Fix variant class link

As reported on https://github.com/godotengine/godot/issues/3488

reference/variant_class.rst

@@ -37,7 +37,7 @@ of C++ with little effort. Become a friend of Variant today.
 References:
 ~~~~~~~~~~~
 
--  `core/variant.h <https://github.com/godotengine/godotengine/godot/blob/master/core/variant.h>`__
+-  `core/variant.h <https://github.com/godotengine/godot/blob/master/core/variant.h>`__
 
 Dictionary and Array
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