docs: fix comment for getHighestFrame

[wy-luke]

I assume that the result of getHighestFrame() should be the frame, rather the frame rate.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/14975/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14975/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14975/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14975/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14975/merge
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https://nme.babylonjs.com/?snapshot=refs/pull/14975/merge

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https://playground.babylonjs.com/?snapshot=refs/pull/14975/merge#BCU1XR#0

[bjsplat]

WebGL2 visualization test reporter:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14975/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU (Experimental)
Important - these might fail sporadically. This is an optional test.

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14975/merge/testResults/webgpuplaywright/index.html