compiler.units: fix modify-code-heap example.
compiler.units: fix modify-code-heap example.

Make sure ``foo`` has the same stack effect as the quotation that is
set on ``foo`` with ``modify-code-heap``.  The symbol had a stack
effect of ( -- x ) since it pushes itself when it is called. The
quotation in the example had stack effect ( -- ).

bpollack/factor,AlexIljin/factor,bpollack/factor,tgunr/factor,mrjbq7/factor,factor/factor,AlexIljin/factor,mrjbq7/factor,AlexIljin/factor,mrjbq7/factor,AlexIljin/factor,factor/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,tgunr/factor,tgunr/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,mrjbq7/factor,factor/factor,factor/factor,tgunr/factor,factor/factor,tgunr/factor,bpollack/factor,tgunr/factor,AlexIljin/factor,bpollack/factor,factor/factor,bpollack/factor