Add missing remove_destination option to a doc [ci skip]

[y-yagi]

Also, fixed the argument name to be the same as the actual argument name.
Ref:

fileutils/lib/fileutils.rb

Line 474 in 64e2750

def copy_entry(src, dest, preserve = false, dereference_root = false, remove_destination = false)

[hsbt]

Thanks! I will backport this to the ruby core repository too.