Change Ruby example to print JSON instead of object (graphql#528)

* Change Ruby example to print JSON instead of object

This commit converts the result of `Schema.execute` to `json`
before printing it renders a more useful result:
`{"data":{"hello":"Hello world!"}}` rather than just the
object `#<GraphQL::Query::Result:0x00007f893535cf80>`.

* oneline

Author: neudabei

site/code/index.html.js

@@ -363,7 +363,7 @@ Schema = GraphQL::Schema.define do
   query QueryType
 end
 
-puts Schema.execute('{ hello }')
+puts Schema.execute('{ hello }').to_json
 \`\`\`
 
 There are also nice bindings for Relay and Rails.