Fix casing for `_on_button_pressed()` definition

[aj-n]

Changing the casing of _on_Button_pressed() to be the correct _on_button_pressed(). May have been changed to all lowercase in 4.0 release?

With default casing, get this error: E 0:00:02:0994 emit_signalp: Error calling from signal 'pressed' to callable: 'Sprite2D(Sprite.gd)::_on_button_pressed': Method not found. <C++ Source> core/object/object.cpp:1058 @ emit_signalp()