Add link to PR in CHANGES.md

Author: lilleyse

CHANGES.md

@@ -11,7 +11,7 @@
 
 #### Fixes :wrench:
 
-- Fixed parsing content bounding volumes contained in 3D Tiles 1.1 subtree files.
+- Fixed parsing content bounding volumes contained in 3D Tiles 1.1 subtree files. [#12972](https://github.com/CesiumGS/cesium/pull/12972)
 - Fixes an event bug following recent changes, where adding a new listener during an event callback caused an infinite loop. [#12955](https://github.com/CesiumGS/cesium/pull/12955)
 - Fix issues with label background when updating properties while `label.show` is `false`. [#12138](https://github.com/CesiumGS/cesium/issues/12138)
 - Improved performance of `scene.drillPick`. [#12916](https://github.com/CesiumGS/cesium/pull/12916)