Grammar (#73)

Author: f3ath

CONTRIBUTING.md

@@ -4,6 +4,6 @@ Thank you for you interest! Here are some key guidelines.
 ## PSR
 We want to follow the latest actual [PSR](http://www.php-fig.org/psr/) standards.
 
-## Tests are documentation
+## Tests are the documentation
 The use cases and library's API must be expressed in a form of tests. We use PHPUnit, but it does not mean all tests
 are expected to be unit tests. No logic must be written without a supporting test.