chore(doc): fix url

readme.md

@@ -3,14 +3,19 @@
 [![NPM version][npm-image]][npm-url]
 [![Test coverage][codecov-image]][codecov-url]
 [![npm download][download-image]][download-url]
-[![Last commit](https://badgen.net/github/last-commit/supperchong/egg-apollo-server)](https://github.com/supperchong/egg-apollo-server)
-[![License](https://badgen.net/github/license/supperchong/egg-apollo-server)](https://github.com/supperchong/egg-apollo-server/blob/master/LICENSE)
+[![Last commit][commit-image]][commit-url]
+[![License][license-image]][license-url]
+
 [npm-image]: https://img.shields.io/npm/v/egg-apollo-server.svg?style=flat-square
 [npm-url]: https://npmjs.org/package/egg-apollo-server
 [codecov-image]: https://codecov.io/gh/supperchong/egg-apollo-server/branch/master/graphs/badge.svg
 [codecov-url]: https://codecov.io/gh/supperchong/egg-apollo-server
 [download-image]: https://img.shields.io/npm/dm/egg-apollo-server.svg?style=flat-square
 [download-url]: https://npmjs.org/package/egg-apollo-server
+[commit-image]: https://badgen.net/github/last-commit/supperchong/egg-apollo-server
+[commit-url]: https://github.com/supperchong/egg-apollo-server
+[license-image]: https://badgen.net/github/license/supperchong/egg-apollo-server
+[license-url]: https://github.com/supperchong/egg-apollo-server/blob/master/LICENSE
 
 
 ## The [apollo-server-koa](https://github.com/apollographql/apollo-server/tree/master/packages/apollo-server-koa) plugin in egg.