ctdb-doc: Fix shellcheck warning in example NFS ganesha call-out

ctdb-doc: Fix shellcheck warning in example NFS ganesha call-out

In ctdb/doc/examples/nfs-ganesha-callout line 216:
for node in `ls ${GANSTATEDIR}`; do
            ^-- SC2045: Iterating over ls output is fragile. Use globs.
            ^-- SC2006: Use $(..) instead of legacy `..`.
                ^-- SC2086: Double quote to prevent globbing and word splitting.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>