docs: fix coveralls badge

README.md

@@ -7,7 +7,7 @@ The fastest JSON Schema validator for Node.js and browser. Supports draft-04/06/
 [![Build Status](https://travis-ci.org/ajv-validator/ajv.svg?branch=master)](https://travis-ci.org/ajv-validator/ajv)
 [![npm](https://img.shields.io/npm/v/ajv.svg)](https://www.npmjs.com/package/ajv)
 [![npm downloads](https://img.shields.io/npm/dm/ajv.svg)](https://www.npmjs.com/package/ajv)
-[![Coverage Status](https://coveralls.io/repos/ajv-validator/ajv/badge.svg?branch=master&service=github)](https://coveralls.io/github/ajv-validator/ajv?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/ajv-validator/ajv/badge.svg?branch=master)](https://coveralls.io/github/ajv-validator/ajv?branch=master)
 [![Gitter](https://img.shields.io/gitter/room/ajv-validator/ajv.svg)](https://gitter.im/ajv-validator/ajv)
 [![GitHub Sponsors](https://img.shields.io/badge/$-sponsors-brightgreen)](https://github.com/sponsors/epoberezkin)