doc: fix typo in fs setacl

Fix typo in the dropbox section where 'l' was referred
to as 'read', not 'lookup'.

Change-Id: I6429c125f0561a1b5d4e7816930988ac1b347be7
Reviewed-on: http://gerrit.openafs.org/10750
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>