Fix a/an typos in doxygen and other comments

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