Fix a property reference in EditorSpinSlider documentation

[YuriSizov]

A regression from one of the recent changes. Basically, we don't seem to validate at any point if a class member referenced in the class documentation is local or inherited but overridden. When it's an overridden reference, the link should be done to the owning class, but all our checks seem to be satisfied with an empty overrides record:

<member name="step" type="float" setter="set_step" getter="get_step" overrides="Range" default="1.0" />

This breaks when building online docs, however, since overridden properties do not generate any sections in the class reference and so don't generate links.

/home/yuris/projects/godot-docs/classes/class_editorspinslider.rst:24: WARNING: undefined label: class_editorspinslider_property_step                                                                      
/home/yuris/projects/godot-docs/classes/class_editorspinslider.rst:24: WARNING: undefined label: class_editorspinslider_property_step

Surprising that this is a rare issue. Ideally I'd want to fix the validation, but I'll have to do this later. Fixing the docs themselves for now should be enough.

[akien-mga]

Thanks!

[mhilbrunner]

This was originally changed here and cherry-picked into 4.1 here, so this fix needs to be cherry-picked to 4.1 as well.

[akien-mga]

Cherry-picked for 4.1.5.