s/room/rooms (#1215)

* s/room/rooms

Fixes #979

* Create 1215.clarification

changelogs/client_server/newsfragments/1215.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

data/api/client-server/joining.yaml

@@ -111,7 +111,7 @@ paths:
     post:
       summary: Start the requesting user participating in a particular room.
       description: |-
-        *Note that this API takes either a room ID or alias, unlike* `/room/{roomId}/join`.
+        *Note that this API takes either a room ID or alias, unlike* `/rooms/{roomId}/join`.
 
         This API starts a user participating in a particular room, if that user
         is allowed to participate in that room. After this call, the client is