docs(firestore): remove await before transaction.update (#5273)

Since it does not return a promise, there is no need to await it. 
This could lead to a confusion among users since `await` usually indicates a promise.

Author: TPXP

docs/firestore/usage/index.md

@@ -546,7 +546,7 @@ function onPostLike(postId) {
       throw 'Post does not exist!';
     }
 
-    await transaction.update(postReference, {
+    transaction.update(postReference, {
       likes: postSnapshot.data().likes + 1,
     });
   });