Merge pull request apollographql#3375 from crash7/fix-add-missing-import

Fix: add missing import in caching update docs

Author: Sashko Stubailo

docs/source/advanced/caching.md

@@ -303,6 +303,8 @@ mutate({
 Using `update` gives you full control over the cache, allowing you to make changes to your data model in response to a mutation in any way you like. `update` is the recommended way of updating the cache after a query. It is explained in full [here](../api/react-apollo.html#graphql-mutation-options-update).
 
 ```javascript
+import CommentAppQuery from '../queries/CommentAppQuery';
+
 const SUBMIT_COMMENT_MUTATION = gql`
   mutation submitComment($repoFullName: String!, $commentContent: String!) {
     submitComment(