fix a typo (missing "a") (elixir-lang#785)

Author: crenwick

getting-started/mix-otp/agent.markdown

@@ -9,7 +9,7 @@ title: Agent
 
 In this chapter, we will create a module named `KV.Bucket`. This module will be responsible for storing our key-value entries in a way it can be read and modified by other processes.
 
-If you have skipped the Getting Started guide or if you have read it long ago, be sure to re-read the chapter about [Processes](/getting-started/processes.html). We will use it as starting point.
+If you have skipped the Getting Started guide or if you have read it long ago, be sure to re-read the chapter about [Processes](/getting-started/processes.html). We will use it as a starting point.
 
 ## The trouble with state