MDL-80820 various: fix incorrect phpdocs

lib/oauthlib.php

@@ -639,7 +639,7 @@ abstract class oauth2_client extends curl {
      * @param string $url The URL to request
      * @param array $options
      * @param mixed $acceptheader mimetype (as string) or false to skip sending an accept header.
-     * @return bool
+     * @return string
      */
     protected function request($url, $options = array(), $acceptheader = 'application/json') {
         $murl = new moodle_url($url);