message: Fix closing quote in modular file system warning
  With current `tf-nightly`, these warnings render as:
  Using modular file system for 'gs. Please switch to [...]
  This patch adds the missing closing quote in (e.g.) `'gs'`.
diff: @@ -644,7 +644,7 @@ struct Register {
  string load_plugin = env_value ? absl::AsciiStrToLower(env_value) : "";
  if (load_plugin == "true" || load_plugin == "1") {
  // We don't register the static filesystem and wait for SIG IO one
  - LOG(WARNING) << "Using modular file system for '" << scheme << "."
  + LOG(WARNING) << "Using modular file system for '" << scheme << "'."
  << " Please switch to tensorflow-io"
  << " (https://github.com/tensorflow/io) for file system"
  << " support of '" << scheme << "'.";
  