Update README.md

README.md

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 [![Build Status](https://travis-ci.org/coreos/flannel.png?branch=master)](https://travis-ci.org/coreos/flannel)
 
-Flannel is a simple and easy way to configure layer 3 network fabric designed for Kubernetes.
+Flannel is a simple and easy way to configure a layer 3 network fabric designed for Kubernetes.
 
 ## How it works