c++: Fix docs on assignment of virtual bases [PR60318]

c++: Fix docs on assignment of virtual bases [PR60318]

The description of behaviour is incorrect, the virtual base gets
assigned before entering the bodies of A::operator= and B::operator=,
not after.

The example is also ill-formed (passing a string literal to char*) and
undefined (missing return from Base::operator=).

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
gcc/ChangeLog:

	PR c++/60318
	* doc/trouble.texi (Copy Assignment): Fix description of
	behaviour and fix code in example.