Fix small typo in the Readme

Author: lucascourot

README.md

@@ -2,7 +2,7 @@
 
 This tool allows you to manage your ignored parameters when running a composer
 install or update. It works when storing the parameters in a Yaml file under
-a a single top-level key (named ``parameters`` by default). Other keys are
+a single top-level key (named ``parameters`` by default). Other keys are
 copied without change.
 
 [![Build Status](https://travis-ci.org/Incenteev/ParameterHandler.png)](https://travis-ci.org/Incenteev/ParameterHandler)