Add missing RFC link in UPGRADING [skip ci]

see 14fc50e

Author: TimWolla

UPGRADING

@@ -246,9 +246,11 @@ PHP 8.5 UPGRADE NOTES
 
 - Core:
   . get_error_handler() allows retrieving the current user-defined error handler
-    function
+    function.
+    RFC: https://wiki.php.net/rfc/get-error-exception-handler
   . get_exception_handler() allows retrieving the current user-defined exception
-    handler function
+    handler function.
+    RFC: https://wiki.php.net/rfc/get-error-exception-handler
 
 - Curl:
   . curl_multi_get_handles() allows retrieving all CurlHandles current