chore(docs): grammatical tweak to page query docs (#12107)

small grammar change

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Author: Kogmo

docs/docs/page-query.md

@@ -2,7 +2,7 @@
 title: Querying data in pages with GraphQL
 ---
 
-Gatsby's `graphql` tag enables page components to retrieve data via GraphQL query.
+Gatsby's `graphql` tag enables page components to retrieve data via a GraphQL query.
 
 In this guide, you will learn [how to use the `graphql` tag](/docs/page-query#add-the-graphql-query) in your pages, as well as go a little deeper into [how the `graphql` tag works](/docs/page-query#how-does-the-graphql-tag-work).