Fix contributing headers

Author: jaime-cespedes-sisniega

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # How to contribute
 
-## Adding feature or solving a bug.
+## Adding a feature or solving a bug
 
 Recommended steps for first time contributors:
 
@@ -24,7 +24,7 @@ Recommended steps for first time contributors:
    ```
 5. Create pull request to the original repository.
 
-## Reporting bugs.
+## Reporting a bug
 
 1. Check that there is not an [issue](https://github.com/IFCA/frouros/issues) that currently highlights the bug or a [pull request](https://github.com/IFCA/frouros/pulls) that solves it.
 2. Create an [issue](https://github.com/IFCA/frouros/issues/new) in GitHub.