add 'use strict' in documentation for ES6 classes

[gh-amistry]

Examples in the "Events" section of the documentation relating to newly introduced ES6 functionality, like classes, don't work unless strict mode is specified, but 'use strict'doesn't appear in the example.

My suggestion is to include 'use strict' in these examples so that the code can simply be copied and pasted as-is and run as working examples.

[jasnell]

SGTM. Interested in submitting a PR? :-)

[evanlucas]

IIRC that will no longer be an issue once v6 drops, but a good idea IMO

[gh-amistry]

Where do I find this content in the repo?
https://nodejs.org/dist/latest-v5.x/docs/api/events.html

[phillipj]

@gh-amistry https://github.com/nodejs/node/blob/master/doc/api/events.md :)

[gh-amistry]

@phillipj Thanks!

[phillipj]

@gh-amistry np! I'm closing this issue for now, but don't hesitate to comment here if you've got any questions. See you in the node repo.