Update introduction-to-mix.markdown (elixir-lang#1330)

Consistent with other examples.

Author: gkostin1966

getting-started/mix-otp/introduction-to-mix.markdown

@@ -206,16 +206,19 @@ Mix also generated a file named `test/test_helper.exs` which is responsible for
 ExUnit.start()
 ```
 
-This file will be required by Mix every time before we run our tests. We can run tests with`mix test`:
+This file will be required by Mix every time before we run our tests. We can run tests with:
 
-Compiled lib/kv.ex
-Generated kv app
-..
+```console
+$ mix test
+Compiled lib/kv.ex
+Generated kv app
+..
 
-Finished in 0.04 seconds
-1 doctest, 1 test, 0 failures
+Finished in 0.04 seconds
+1 doctest, 1 test, 0 failures
 
-Randomized with seed 540224
+Randomized with seed 540224
+```
 
 Notice that by running `mix test`, Mix has compiled the source files and generated the application manifest once again. This happens because Mix supports multiple environments, which we will discuss later in this chapter.