webdriver: readme: fix link to image

Source-Repo: https://github.com/mozilla/webdriver-rust
Source-Revision: 847188b1c3ad8ecf5062e61cde1763f5cedbac93

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extra : subtree_source : http%3A//tristan.corp.lon2.mozilla.com%3A8000
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testing/webdriver/README.md

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-# webdriver Rust library [![Build status](https://travis-ci.org/mozilla/webdriver-rust.svg?branch=master)](https://travis-ci.org/mozilla/webdriver-rust) [![Documentation](https://docs.rs/mio/badge.svg)](https://docs.rs/webdriver/)
+# webdriver Rust library [![Build status](https://travis-ci.org/mozilla/webdriver-rust.svg?branch=master)](https://travis-ci.org/mozilla/webdriver-rust) [![Documentation](https://docs.rs/webdriver/badge.svg)](https://docs.rs/webdriver/)
 
 As of right now this is an implementation
 for the server side of the WebDriver API in Rust,