Switch %CLIENT_MAJOR_VERSION% in /login endpoint for current /login v…

…ersion

This was left over from the days of all CS API endpoints sharing
the same major spec release version.

content/application-service-api.md

@@ -342,7 +342,7 @@ log in without needing the user's password. This is achieved by including the
 
 {{% added-in v="1.2" %}}
 
-    POST /_matrix/client/%CLIENT_MAJOR_VERSION%/login
+    POST /_matrix/client/v3/login
     Authorization: Bearer YourApplicationServiceTokenHere
 
     Content: