Improve installation instructions, fixes rmosolgo#4472

Author: rmosolgo

guides/index.html

@@ -12,7 +12,7 @@ <h2>Install the Gem</h2>
       <div class="teaser">
 {% highlight bash %}
 # Download the gem:
-gem install graphql
+bundle add graphql
 # Setup with Rails:
 rails generate graphql:install
 {% endhighlight %}