fix auth documentation on auth

doc/shellmatta_auth.dox

@@ -76,9 +76,9 @@
         shellmatta_retCode_t custom_auth_check(const uint32_t userId, const char* password) {
             /‌/ Check if the passed userID matches the passed password.
             if (password_matches()) {
-                return userId;
+                return SHELLMATTA_OK;
             }
-            return 0;
+            return SHELLMATTA_ERROR;
         }
 
 */