Auto merge of #17452 - larsbergstrom:readme_change_for_backout, r=met…

…ajack

Commiting a trivial README.md change for the Firefox sheriffs to backout

This change is intended to be used to test the backout scripts from the Firefox side. See the following mail thread for more details:
https://groups.google.com/forum/#!topic/mozilla.dev.servo/j4C7U_H_T2A

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17452)
<!-- Reviewable:end -->

README.md

@@ -2,7 +2,7 @@
 
 [![Linux Build Status](https://img.shields.io/travis/servo/servo/master.svg?label=Linux%20build)](https://travis-ci.org/servo/servo)  [![Windows Build Status](https://img.shields.io/appveyor/ci/servo/servo/master.svg?label=Windows%20build)](https://ci.appveyor.com/project/servo/servo/branch/master)  [![Changelog #228](https://img.shields.io/badge/changelog-%23228-9E978E.svg)](https://changelog.com/podcast/228)
 
-Servo is a prototype web browser engine written in the
+Servo is a web browser engine written in the
 [Rust](https://github.com/rust-lang/rust) language. It is currently developed on
 64bit OS X, 64bit Linux, and Android.