doc: fix typo in fs setacl

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

Reviewed-on: http://gerrit.openafs.org/10750
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit eb3f8c99edcf005dfba836ae9ab1d3102f162c2a)

Change-Id: I177e6975c19d176e5a7277d0b458e36b4e5f99f9
Reviewed-on: http://gerrit.openafs.org/10751
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>