Make StringSliceFlag usage text consistent

This fixes StringSliceFlag default usage text to be consistent with
IntegerSliceFlag.

Before:

   -f []	`-f option -f option` port forwarding rules

After:

   -f '-f option -f option'	port forwarding rules

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #83 from penberg/string-slice-usage-text

Make StringSliceFlag usage text consistent