Update coverage badge in README.md

Author: fluentfuture

README.md

@@ -1,6 +1,6 @@
 Disclaimer: This is not an official Google product.
 
-# Mug (![Coverage](.github/badges/jacoco.svg))
+# Mug (![Coverage](.github/badges/mug-coverage.svg))
 A small Java 8+ string processing and streams library ([javadoc](http://google.github.io/mug/apidocs/index.html)), widely used in Google's internal Java codebase, with **0 deps** (Proto, BigQuery, Guava addons are in separate artifacts). ![](https://travis-ci.org/google/mug.svg?branch=master)
 
 ## Highlights