Grammar fix: add missing "an"

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

@@ -408,6 +408,6 @@ It depends. It is ok to rely on shared global state as long as we depend only on
 
 Similar reasoning should be applied to our bucket supervisor. Although multiple registries may start buckets on the shared bucket supervisor, those buckets and registries are isolated from each other. We would only run into concurrency issues if we used a function like `Supervisor.count_children(KV.Bucket.Supervisor)` which would count all buckets from all registries, potentially giving different results when tests run concurrently.
 
-Since we have relied only on a non-shared partition of the bucket supervisor so far, we don't need to worry about concurrency issues in our test suite. In case it ever becomes a problem, we can start a supervisor per test and pass it as argument to the registry `start_link` function.
+Since we have relied only on a non-shared partition of the bucket supervisor so far, we don't need to worry about concurrency issues in our test suite. In case it ever becomes a problem, we can start a supervisor per test and pass it as an argument to the registry `start_link` function.
 
 Now that our application is properly supervised and tested, let's see how we can speed things up.