Fix a typo in logstash docs

This fixes a typo in the logstash module documentation.  The
original documentation said "mulitple" which is an unword.

Change-Id: Ied3e3a3356590ec188f8f2176ebfaa0169957fcd
Reviewed-on: https://review.openstack.org/34756
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins