message: Fix typo in description of INTERNAL error code.
diff: @@ -119,7 +119,7 @@ enum Code {
  // Operation is not implemented or not supported/enabled in this service.
  UNIMPLEMENTED = 12;
  - // Internal errors. Means some invariants expected by underlying
  + // Internal errors. Means some invariant expected by the underlying
  // system has been broken. If you see one of these errors,
  // something is very broken.
  INTERNAL = 13;
  