relocatable-shell: Update suggested usage in maintainer documentation.

relocatable-shell: Update suggested usage in maintainer documentation.

Instead of suggesting an inline usage of "sed", that would have to be
cut-and-pasted for every directory to be relocated, suggest a shell
function.

Make the example obviously an example, whereas previously it looked like
it might be literal text.

Thanks to Reuben Thomas <rrt@sc3d.org> for pointing out these issues.  See
http://lists.gnu.org/archive/html/bug-gnulib/2014-01/msg00039.html for
further context.