Generic6DOFJoint C# highlight correction

### Tested versions

Reproducible in Godot 4.6.stable.

### System information

Linux, Godot 4.6.stable.mono

### Issue description

4.6 Stable Mono version of Godot has issues with `Generic6DOFJoint3D` highlighting. The actual name of the class in Godot mono is `Generic6DoFJoint3D` with a lowercase 'O' (which is weird and not inline with the class specification in docs), but editor highlights only `Generic6DOFJoint3D` with a capital 'O'.

I guess we either need to correct highlighting or correct the class name so it's inline with other classes like World3D, with multiple capital letters.

### Steps to reproduce

Create the script, open the editor and add a property with Generic6DoFJoint3D type to your class.

### Minimal reproduction project (MRP)

Not required.