Fix comment typo in rsa_sign_wrap()

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 2908451..c30b521 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -222,7 +222,7 @@
     if( sig_size < *sig_len )
         return( MBEDTLS_ERR_PK_BUFFER_TOO_SMALL );
 
-    /* mbedtls_pk_write_pubkey() expects a full PK context;
+    /* mbedtls_pk_write_key_der() expects a full PK context;
      * re-construct one to make it happy */
     key.pk_info = &pk_info;
     key.pk_ctx = ctx;