Intersphinx role, simplify role_name check

Co-authored-by: Takeshi KOMIYA <i.tkomiya@gmail.com>

sphinx/ext/intersphinx.py

@@ -478,7 +478,7 @@ class IntersphinxDispatcher(CustomReSTDispatcher):
 
     def role(self, role_name: str, language_module: ModuleType, lineno: int, reporter: Reporter
              ) -> Tuple[RoleFunction, List[system_message]]:
-        if len(role_name) > 9 and role_name.startswith('external') and role_name[8] in ':+':
+        if len(role_name) > 9 and role_name.startswith(('external:', 'external+')):
             return IntersphinxRole(role_name), []
         else:
             return super().role(role_name, language_module, lineno, reporter)