Use better key

translations/en.json

@@ -613,5 +613,5 @@
   "public": "Public",
   "public_blurb": "This community can be federated to other instances and be posted/commented in by their users.",
   "banned_from_community_blurb": "<1>Banned:</1> You cannot post, comment, or vote in this community. However, you are still free to browse.",
-  "create_form_user_matrix_blurb": "This user already has a matrix account you can message at <1>{{matrix_id}}</1>."
+  "private_message_form_user_matrix_blurb": "This user already has a matrix account you can message at <1>{{matrix_id}}</1>."
 }