message: fix link to runtime docs
  Description: Looks like the runtime docs moved under operations/. The PR fixes the link.
  Risk Level: low
  Testing: existing
  Docs Changes: this
  Release Notes: n/a
diff: @@ -220,7 +220,7 @@ void MessageUtil::checkForUnexpectedFields(const Protobuf::Message& message,
  } else {
  const char fatal_error[] =
  " If continued use of this field is absolutely necessary, see "
  - "https://www.envoyproxy.io/docs/envoy/latest/configuration/runtime"
  + "https://www.envoyproxy.io/docs/envoy/latest/configuration/operations/runtime"
  "#using-runtime-overrides-for-deprecated-features for how to apply a temporary and "
  "highly discouraged override.";
  throw ProtoValidationException(err + fatal_error, message);
  