Update Link-Check CI and Replace a Dead Link (#3456)

.github/workflows/docs-linkcheck.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ${{matrix.os}}
     strategy:
       matrix:
-        python-version: [3.7]
+        python-version: [3.9]
         os: [ubuntu-latest]
       fail-fast: False
     steps:

telegram/ext/_commandhandler.py

@@ -70,7 +70,7 @@ class CommandHandler(BaseHandler[Update, CCT]):
     Args:
         command (:obj:`str` | Collection[:obj:`str`]):
             The command or list of commands this handler should listen for. Case-insensitive.
-            Limitations are the same as described `here <https://core.telegram.org/bots#commands>`_
+            Limitations are the same as for :attr:`telegram.BotCommand.command`.
         callback (:term:`coroutine function`): The callback function for this handler. Will be
             called when :meth:`check_update` has determined that an update should be processed by
             this handler. Callback signature::