Update tagline in the page title (#2365)

Keep the title in sync with the body. 

Thanks @Keweiqu for noticing this 🙏

Author: martinbonnin

src/pages/index.mdx

@@ -1,5 +1,5 @@
 ---
-title: GraphQL | A query language for your API
+title: GraphQL | The query language for modern APIs
 ---
 
 export { IndexPage as default } from '../components/index-page'