Fix various typos in docs (#17114)

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

docs/admin_api/room_membership.md

@@ -1,6 +1,6 @@
 # Edit Room Membership API
 
-This API allows an administrator to join an user account with a given `user_id`
+This API allows an administrator to join a user account with a given `user_id`
 to a room with a given `room_id_or_alias`. You can only modify the membership of
 local users. The server administrator must be in the room and have permission to
 invite users.