Add Ruby syntax highlighting to "Multiple Trackers" example in README

[hoffm]

No description provided.

[mongoid-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#242](https://github.com/mongoid/mongoid-history/pull/242): Add ruby syntax highlighting to "multiple trackers" example in readme - [@hoffm](https://github.com/hoffm).

Generated by 🚫 danger

[mongoid-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#242](https://github.com/mongoid/mongoid-history/pull/242): Add ruby syntax highlighting to "multiple trackers" example in readme - [@hoffm](https://github.com/hoffm).

Generated by 🚫 danger

[coveralls]

Coverage remained the same at 99.417% when pulling dd8ff9a on hoffm:patch-1 into 6616cea on mongoid:master.

[coveralls]

Coverage remained the same at 99.417% when pulling dd8ff9a on hoffm:patch-1 into 6616cea on mongoid:master.

[dblock]

👍merged

If you're looking for ways to contribute, #238 is probably doable, starting with travis CI for mongoid 7.

[hoffm]

@dblock Thanks! I haven't used this library yet—came upon it doing some research—but if I do I'll check back for ways to contribute :)