message: Fix gitter badge copy-paste error
diff: [travis-badge]: https://travis-ci.org/brendanzab/pikelet.svg?branch=master
  [travis-url]: https://travis-ci.org/brendanzab/pikelet
  -[gitter-badge]: https://badges.gitter.im/gluon-lang/gluon.svg
  +[gitter-badge]: https://badges.gitter.im/pikelet-lang/pikelet.svg
  [gitter-lobby]: https://gitter.im/pikelet-lang/Lobby
  This is an implementation of a small [dependently typed][dependent-type-wikipedia]
  