Wrong formula in Matrix3.fromHeadingPitchRoll()

[emackey]

Reported by @zimengyang in this comment

I double checked Cesium's implementation in Matrix3.fromHeadingPitchRoll(), there is a bug here:
Cesium/Core/Matrix3.js, line 321, formula here is incorrect,

The correct formula is:
var m12 = -sinPhi * cosPsi + cosPhi * sinTheta * sinPsi;

Instead of
var m12 = -sinTheta * cosPhi + cosPhi * sinTheta * sinPsi;

[pjcozzi]

@rahwang could you open a pull request with the fix?

[rahwang]

@pjcozzi yep! On it!