Fix lcb_set_store_callback() signature in man page

Change-Id: I03554ad98de1fd51589870d18b9e2be59ed23909
Reviewed-on: http://review.couchbase.org/25295
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>