GODRIVER-1164 update ChangeStream docs example

Change-Id: I94d2f47c3fa0d7571420117c1e1ffce4b0827114

Author: iwysiu

examples/documentation_examples/examples.go

@@ -2002,9 +2002,8 @@ func ChangeStreamExamples(t *testing.T, db *mongo.Database) {
 		ok := original.Next(ctx)
 		require.True(t, ok)
 
-		next := original.Current
 		// Start Changestream Example 3
-		resumeToken := next.Lookup("_id").Document()
+		resumeToken := original.ResumeToken()
 
 		cs, err := coll.Watch(ctx, mongo.Pipeline{}, options.ChangeStream().SetResumeAfter(resumeToken))
 		require.NoError(t, err)