Clarify _bridge_alice

Co-authored-by: Tulir Asokan <tulir@maunium.net>

proposals/2778-appservice-login.md

@@ -25,7 +25,7 @@ Example request
   "type": "m.login.application_service",
   "identifier": {
     "type": "m.id.user",
-    "user": "alice"
+    "user": "_bridge_alice"
   }
 }
 ```
@@ -78,4 +78,4 @@ This can safely be ignored or used, but is an extra token hanging around.
 ## Unstable prefix
 
 Implementations should use `uk.half-shot.msc2778.login.application_service` for `type` given in the
-`POST /login` until this lands in a released version of the specification.
+`POST /login` until this lands in a released version of the specification.