Refine add.md for Practice Exercises and fix minor typo (#577)

Author: tasxatzial

building/tracks/practice-exercises/add.md

@@ -45,7 +45,7 @@ Once the scaffolded files have been created, you'll then have to:
 ### Add tests
 
 A key part of adding an exercise is adding tests.
-Roughly speaking, there are two options when adding tests for one of the above exercises:
+Roughly speaking, there are two options when adding tests for a Practice Exercise:
 
 1. Implement the tests from scratch, using the test cases from the exercise's `canonical-data.json` file as found in the [problem-specifications repo](https://github.com/exercism/problem-specifications).
 2. Port the tests from another track's implementation (tip: go to `https://exercism.org/exercises/<slug>` to get an overview of which tracks have implemented a specific exercise).
@@ -128,7 +128,7 @@ First, make sure you have the latest version of [`configlet`](/docs/building/con
 bin/fetch-configlet
 ```
 
-The run [the linter](/docs/building/configlet/lint) by running:
+Then run [the linter](/docs/building/configlet/lint) by running:
 
 ```shell
 bin/configlet lint