Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index d60ca0d..5554720 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -86,7 +86,7 @@
 
 #if defined(MBEDTLS_BIGNUM_C)
 /**
- * \brief           Write a arbitrary-precision number (#MBEDTLS_ASN1_INTEGER)
+ * \brief           Write an arbitrary-precision number (#MBEDTLS_ASN1_INTEGER)
  *                  in ASN.1 format.
  *
  * \note            This function works backwards in data buffer.
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 2bf24a0..3e53c33 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -1110,7 +1110,7 @@
  * Include backtrace information with each allocated block.
  *
  * Requires: MBEDTLS_MEMORY_BUFFER_ALLOC_C
- *           GLIBC-compatible backtrace() an backtrace_symbols() support
+ *           GLIBC-compatible backtrace() and backtrace_symbols() support
  *
  * Uncomment this macro to include backtrace information
  */
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index 612061d..fbf938d 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -199,7 +199,7 @@
 int mbedtls_md_setup( mbedtls_md_context_t *ctx, const mbedtls_md_info_t *md_info, int hmac );
 
 /**
- * \brief           This function clones the state of an message-digest
+ * \brief           This function clones the state of a message-digest
  *                  context.
  *
  * \note            You must call mbedtls_md_setup() on \c dst before calling
diff --git a/include/mbedtls/ripemd160.h b/include/mbedtls/ripemd160.h
index 9132a83..397ec84 100644
--- a/include/mbedtls/ripemd160.h
+++ b/include/mbedtls/ripemd160.h
@@ -66,7 +66,7 @@
 void mbedtls_ripemd160_free( mbedtls_ripemd160_context *ctx );
 
 /**
- * \brief          Clone (the state of) an RIPEMD-160 context
+ * \brief          Clone (the state of) a RIPEMD-160 context
  *
  * \param dst      The destination context
  * \param src      The context to be cloned
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index e43ef96..002551f 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -479,7 +479,7 @@
  *             the current function does not have access to them,
  *             and therefore cannot check them. See mbedtls_rsa_complete().
  *             If you want to check the consistency of the entire
- *             content of an PKCS1-encoded RSA private key, for example, you
+ *             content of a PKCS1-encoded RSA private key, for example, you
  *             should use mbedtls_rsa_validate_params() before setting
  *             up the RSA context.
  *             Additionally, if the implementation performs empirical checks,
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 5099cfb..7d99720 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -756,7 +756,7 @@
  *                 for the associated \c mbedtls_ssl_get_timer_t callback to
  *                 return correct information.
  *
- * \note           If using a event-driven style of programming, an event must
+ * \note           If using an event-driven style of programming, an event must
  *                 be generated when the final delay is passed. The event must
  *                 cause a call to \c mbedtls_ssl_handshake() with the proper
  *                 SSL context to be scheduled. Care must be taken to ensure