Fix incorrect graphql code block. Update graphql-validations-migratio… (#580)

* Fix incorrect graphql code block. Update graphql-validations-migration-guide.mdx

* Update graphql-validations-migration-guide.mdx

Author: Dimitri POSTOLOV

website/pages/en/release-notes/graphql-validations-migration-guide.mdx

@@ -368,19 +368,17 @@ Also, a GraphQL field selection is only valid if the following is validated:
 Here are a few examples of violations of these rules with the following Schema:
 
 ```graphql
-schema {
-  type Image {
-   url: String!
-  }
+type Image {
+  url: String!
+}
 
-  type User {
-   id: ID!
-   avatar: Image!
-  }
+type User {
+  id: ID!
+  avatar: Image!
+}
 
-  type Query {
-    user: User!
-  }
+type Query {
+  user: User!
 }
 ```