Fix typo in tempest configuration guide
Fix typo in tempest configuration guide

The indefinite article of the word "alternate" had became "a".
Fix it to "an".

Closes-Bug #1486786

Change-Id: I8d66d066f218fdb3cb70e45ebbcc6be6ae2e5925

rakeshmi/tempest,Juniper/tempest,zsoltdudas/lis-tempest,dkalashnik/tempest,openstack/tempest,cisco-openstack/tempest,bigswitch/tempest,dkalashnik/tempest,masayukig/tempest,tonyli71/tempest,tonyli71/tempest,bigswitch/tempest,LIS/lis-tempest,xbezdick/tempest,izadorozhna/tempest,Juniper/tempest,openstack/tempest,cisco-openstack/tempest,vedujoshi/tempest,sebrandon1/tempest,Tesora/tesora-tempest,Tesora/tesora-tempest,sebrandon1/tempest,izadorozhna/tempest,xbezdick/tempest,vedujoshi/tempest,pczerkas/tempest,flyingfish007/tempest,LIS/lis-tempest,rakeshmi/tempest,zsoltdudas/lis-tempest,pczerkas/tempest,flyingfish007/tempest,masayukig/tempest