Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 32578e2..88915a1 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2862,7 +2862,7 @@
         else
         {
             /*
-             * An necessary condition for Y and X = 2Y + 1 to be prime
+             * A necessary condition for Y and X = 2Y + 1 to be prime
              * is X = 2 mod 3 (which is equivalent to Y = 2 mod 3).
              * Make sure it is satisfied, while keeping X = 3 mod 4
              */
diff --git a/library/error.c b/library/error.c
index afad389..630e142 100644
--- a/library/error.c
+++ b/library/error.c
@@ -961,7 +961,7 @@
 #else /* MBEDTLS_ERROR_C */
 
 /*
- * Provide an non-function in case MBEDTLS_ERROR_C is not defined
+ * Provide a dummy implementation when MBEDTLS_ERROR_C is not defined
  */
 void mbedtls_strerror( int ret, char *buf, size_t buflen )
 {
diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 107e912..079526c 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -872,7 +872,7 @@
 {
     void *ctx = mbedtls_calloc( 1, sizeof( psa_key_id_t ) );
 
-    /* no _init() function to call, an calloc() already zeroized */
+    /* no _init() function to call, as calloc() already zeroized */
 
     return( ctx );
 }
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 72351c9..74b2df7 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -174,7 +174,7 @@
 
     *olen = 0;
 
-    /* We're always including an TLS_EMPTY_RENEGOTIATION_INFO_SCSV in the
+    /* We're always including a TLS_EMPTY_RENEGOTIATION_INFO_SCSV in the
      * initial ClientHello, in which case also adding the renegotiation
      * info extension is NOT RECOMMENDED as per RFC 5746 Section 3.4. */
     if( ssl->renego_status != MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS )
diff --git a/library/xtea.c b/library/xtea.c
index 77f6cb6..01478ac 100644
--- a/library/xtea.c
+++ b/library/xtea.c
@@ -1,5 +1,5 @@
 /*
- *  An 32-bit implementation of the XTEA algorithm
+ *  A 32-bit implementation of the XTEA algorithm
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0