Use cp instead of rsync in hack/verify-gendocs.sh

Instead of calling rsync over and over and over and over and over and
over and over and over and over (and probably over) use one `cp`

Before:
real	0m5.247s
user	0m2.294s
sys	0m1.300s

After:
real	0m2.260s
user	0m2.230s
sys	0m0.936s

hack/lib/util.sh

@@ -171,7 +171,11 @@ kube::util::gen-doc() {
   done <"${dest}/.files_generated"
 
   # put the new generated file into the destination
-  find "${tmpdir}" -exec rsync -pt {} "${dest}" \; >/dev/null
+  # the shopt is so that we get .files_generated from the glob.
+  shopt -s dotglob
+  cp -af "${tmpdir}"/* "${dest}"
+  shopt -u dotglob
+
   #cleanup
   rm -rf "${tmpdir}"
 }