Update README.md (#5288)

* Update README.md

Missing curly bracket

* formatting

Author: mklan

packages/gatsby-source-contentful/README.md

@@ -81,7 +81,8 @@ if you have `Product` as one of your content types, you will be able to query it
 like the following:
 
 ```graphql
-allContentfulProduct {
+{
+  allContentfulProduct {
     edges {
       node {
         id