Fix a typo

[tobenaii]

Fixed a typo! :>

[akien-mga]

Thanks for the contribution!

The fix is correct, but there are two issues:

• PRs should be made against the master branch so that the issue is fixed first and foremost for all future Godot versions. Then we cherry-pick relevant changes to stable branches (3.3, 3.2, stable) ourselves.

• The class reference is generated from XML files in the Godot source repository, so it should not be modified here: https://github.com/godotengine/godot/blob/master/modules/csg/doc_classes/CSGMesh3D.xml

As this is a simple change I've batched it with another doc update that I just pushed to Godot's master branch: godotengine/godot@cab5064