Fixed note in CHANGES.md about throttleRequestByServer

[lilleyse]

The note in CHANGES.md about replacing throttleRequestByServer with RequestScheduler.throttleRequest was wrong since that function doesn't exist. The note is corrected.

Brought up on the forum: https://groups.google.com/forum/#!topic/cesium-dev/iN44M7palqw

[cesium-concierge]

@lilleyse, thanks for the pull request! Maintainers, we have a signed CLA from @lilleyse, so you can review this at any time.

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[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!topic/cesium-dev/iN44M7palqw

If this issue affects any of these threads, please post a comment like the following:

The issue at #5935 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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I am a bot who helps you make Cesium awesome! Thanks again.