Fix typo (elixir-lang#1288)

Author: byteallen

getting-started/mix-otp/supervisor-and-application.markdown

@@ -19,7 +19,7 @@ If you have prior programming experience, you may be wondering: "could we just g
 
 Therefore, an Elixir developer prefers to "let it crash" or "fail fast". And one of the most common ways we can recover from a failure is by restarting whatever part of the system that crashed.
 
-For example, when your computer, router, printer, or whatever device is not working properly. How do you often fix it? By restarting it. Once we restart the device, we reset the device back to its initial state, which is well-tested aand guaranteed to work. In Elixir, we apply this same approach to software: whenever a process crashes, we start a new process to perform the same job as the crashed process.
+For example, when your computer, router, printer, or whatever device is not working properly. How do you often fix it? By restarting it. Once we restart the device, we reset the device back to its initial state, which is well-tested and guaranteed to work. In Elixir, we apply this same approach to software: whenever a process crashes, we start a new process to perform the same job as the crashed process.
 
 In Elixir, this is done by a Supervisor. A Supervisor is a process that supervises other processes and restarts them whenever they crash. To do so, Supervisors manage the whole life-cycle of any supervised processes, including startup and shutdown.