Update buffer.md

Author: 38elements

doc/api/core/operators/buffer.md

@@ -134,7 +134,7 @@ File:
 Dist:
 - [`rx.all.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.js)
 - [`rx.all.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.js)
-- [rx.coincidence.js](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.coincidence.js)
+- [`rx.coincidence.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.coincidence.js)
 
 Prerequisites:
 - [`rx.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js) | [`rx.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.compat.js) | [`rx.lite.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.js) | [`rx.lite.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.compat.js)