Bug 1786319 - doc(mochitest) - add a link to the js script

Differential Revision: https://phabricator.services.mozilla.com/D157186

testing/docs/mochitest-plain/index.md

@@ -233,8 +233,8 @@ Right now, useful helpers derived from MochiKit are available in
 
 Although all of Mochikit is available at `testing/mochitest/MochiKit`, only
 include files that you require to minimize test load times. Bug 367569 added
-`sendChar`, `sendKey`, and `sendString` helpers. These are available in
-`testing/mochitest/tests/SimpleTest/EventUtils.js`.
+`sendChar`, `sendKey`, and `sendString` helpers.
+These are available in [`testing/mochitest/tests/SimpleTest/EventUtils.js`](https://searchfox.org/mozilla-central/source/testing/mochitest/tests/SimpleTest/EventUtils.js).
 
 If you need to access some data files from your Mochitest, you can get an URI
 for them by using `SimpleTest.getTestFileURL("relative/path/to/data.file")`.