[llvm-objcopy] Change --only-keep to --only-section

I just hard core goofed when I wrote this and created a different name
for no good reason. I'm failry aware of most "fresh" users of llvm-objcopy
(that is, users which are not using it as a drop in replacement for GNU
objcopy) and can say that only "-j" is being used by such people so this
patch should strictly increase compatibility and not remove it.

Differential Revision: https://reviews.llvm.org/D52180

llvm-svn: 348446