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 8717c8a..55325b4 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2803,7 +2803,7 @@
         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/pk_wrap.c b/library/pk_wrap.c
index e234a8d..aea84e5 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -1469,7 +1469,7 @@
 {
     void *ctx = mbedtls_calloc( 1, sizeof( mbedtls_svc_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_tls12_client.c b/library/ssl_tls12_client.c
index 2405208..4c4302f 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -85,7 +85,7 @@
 
     *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 )