docs(afs): Fixed typo (#2729)

simple -> simply

Author: EthanSK

docs/firestore/collections.md

@@ -217,7 +217,7 @@ export class AppComponent {
 
 **What is it?** - Returns an Observable of `DocumentChangeAction[]` as they occur. Similar to `stateChanges()`, but instead it keeps around the trail of events as an array.
 
-**Why would you use it?** - This method is like `stateChanges()` except it is not ephemeral. It collects each change in an array as they occur. This is useful for ngrx integrations where you need to replay the entire state of an application. This also works as a great debugging tool for all applications. You can simple write `afs.collection('items').auditTrail().subscribe(console.log)` and check the events in the console as they occur.
+**Why would you use it?** - This method is like `stateChanges()` except it is not ephemeral. It collects each change in an array as they occur. This is useful for ngrx integrations where you need to replay the entire state of an application. This also works as a great debugging tool for all applications. You can simply write `afs.collection('items').auditTrail().subscribe(console.log)` and check the events in the console as they occur.
 
 **When would you not use it?** - When you just need a list of data. This is a more advanced usage of AngularFirestore.