Update firestore.dart to fix pagination with document snapshot

When paginating you can either pass an array of field values for. the cursor document, or you can pass a single `DocumentSnapshot` object. The code samples for using a `DocumentSnapshot` call the wrong function, and this PR fixes that.

Author: puf

packages/firebase_snippets_app/lib/snippets/firestore.dart

@@ -929,7 +929,7 @@ class FirestoreSnippets extends DocSnippet {
         final biggerThanSf = db
             .collection("cities")
             .orderBy("population")
-            .startAt([documentSnapshot]);
+            .startAtDocument(documentSnapshot);
       },
       onError: (e) => print("Error: $e"),
     );
@@ -951,7 +951,7 @@ class FirestoreSnippets extends DocSnippet {
         final next = db
             .collection("cities")
             .orderBy("population")
-            .startAfter([lastVisible]).limit(25);
+            .startAfterDocument(lastVisible).limit(25);
 
         // Use the query for pagination
         // ...