Fixed missing 'a' in Controllers section of documentation (#1108)

Minor typo fixed. There was an 'a' missing in the getting started folder
structure section for controllers.

Original: "Controllers are great way..."
Corrected: "Controllers are a great way of grouping together application
logic."

Author: Yoorbo

docs/getting-started/folder-structure.md

@@ -47,7 +47,7 @@ This is where all of your application logic goes.
 
 #### Controllers
 
-Controllers are great way of grouping together application logic. Most controllers have many functions that accept a request and return some sort of response.
+Controllers are a great way of grouping together application logic. Most controllers have many functions that accept a request and return some sort of response.
 
 #### Migrations