Correct signature of __build_class__ (GH-16735)

Python/bltinmodule.c

@@ -260,7 +260,7 @@ error:
 }
 
 PyDoc_STRVAR(build_class_doc,
-"__build_class__(func, name, *bases, metaclass=None, **kwds) -> class\n\
+"__build_class__(func, name, /, *bases, [metaclass], **kwds) -> class\n\
 \n\
 Internal helper function used by the class statement.");