Fix typo in CONTRIBUTING.md (#327)

Author: TimWolla

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # How to contribute
 
-Thanks for contributing to assert! Just follow these single guidelines:
+Thanks for contributing to assert! Just follow these simple guidelines:
 
 - You must use [feature / topic branches](https://git-scm.com/book/en/v2/Git-Branching-Branching-Workflows) to ease the merge of contributions.
 - Coding standard compliance must be ensured before committing or opening pull requests by running `composer assert:cs-fix` or `composer assert:cs-lint` in the root directory of this repository.