[ob] Fix comment in rs6000-nat.c

[Jerome Guitton]

Jerome Guitton (guitton@act-europe.fr):

> 
> bfd_cache_lookup is not used. bfd_default_set_arch_mach is.
> Will commit as obvious.

I forgot the obvious Changelog entry:

2004-05-14  Jerome Guitton  <guitton@gnat.com>

	* rs6000-nat.c: Fix a comment.