Fix a/an typos in doxygen and other comments

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