Fix wrong example for chained tokens in hook_tokens()

In hook_tokens(), the chainable token replacements overwrite all previously set token replacements:

    if ($author_tokens = $token_service->findWithPrefix($tokens, 'author')) {
      $replacements = $token_service->generate(/* snip */);
    }

In practice, such as in node_tokens(), it looks like this:

   if ($author_tokens = $token_service->findWithPrefix($tokens, 'author')) {
      $replacements += $token_service->generate(/* snip */);
    }