Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
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