Fix JSON API sample.

samples/json_api.cr

@@ -4,6 +4,6 @@ require "json"
 # You can easily access the context and set content_type like 'application/json'.
 # Look how easy to build a JSON serving API.
 get "/" do |env|
-  env.set_content_type = "application/json"
+  env.content_type = "application/json"
   {name: "Serdar", age: 27}.to_json
 end