[Doc] Fix incorrect reference to input_pickable

[AThousandShips]

Causes sphinx build on godot-docs to fail

[akien-mga]

How come there is no warning/error when running make_rst.py?

[AThousandShips]

Probably because there is an override in PhysicsBody2D, will look at the docs data

Edit: It doesn't have a label in the rst as it's just an override, but the property technically exists, so it's not possible to directly reference it, but it's there, so the make_rst.py doesn't catch it

Probably some missing check in the properties validation somewhere but I don't know where the specific validation is run, can dig later and see if I can add a safer check there

[akien-mga]

Thanks!

[AThousandShips]

Thank you!

[akien-mga]

Cherry-picked for 4.2.3.
Cherry-picked for 4.1.5.