Update firestore.rules (firebase#629)

Fixing a typo.

Author: mikedory

quickstarts/uppercase-firestore/firestore.rules

@@ -1,8 +1,8 @@
 service cloud.firestore {
   match /databases/{database}/documents {
     match /messages/{message} {
-      // Allow authnticated users to read/write the messages collection
+      // Allow authenticated users to read/write the messages collection
       allow read, write: if request.auth != null;
     }
   }
-}
+}