Merely fixed the capitalization of a word.
## Detailed Description of the Pull Request / Additional comments
- Inside `doc/AddASetting.md`, on line 14, the sentence previously began with the verb "add" with a lowercase alphabet.
- I replaced "add" with "Add".
``` md
[12] ...
[13] 2. Add matching fields to Settings.hpp
[14] - Add getters, setters, the whole drill.
[15] ...
```