Fix typo in docs

[OmarShehata]

In https://cesiumjs.org/Cesium/Build/Documentation/Viewer.html?classFilter=Viewer#selectedEntityChanged

changed:

Gets the event that is raised when the selected entity chages

to:

Gets the event that is raised when the selected entity changes.

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[hpinkos]

👍 thanks @OmarShehata!