Fix typo in Tempest Coding Guide.
Fix typo in Tempest Coding Guide.

"doctstrings" was mis-spelled as "docstings" in
tempest/HACKING.html

Change-Id: I0ae2b84af718cc032cd031612a898895a2b3c21d
Closes-bug: #1484356

flyingfish007/tempest,masayukig/tempest,rakeshmi/tempest,tonyli71/tempest,roopali8/tempest,dkalashnik/tempest,manasi24/tempest,Juniper/tempest,akash1808/tempest,varunarya10/tempest,tonyli71/tempest,dkalashnik/tempest,LIS/lis-tempest,zsoltdudas/lis-tempest,pczerkas/tempest,JioCloud/tempest,sebrandon1/tempest,akash1808/tempest,xbezdick/tempest,vedujoshi/tempest,manasi24/tempest,LIS/lis-tempest,tudorvio/tempest,tudorvio/tempest,Juniper/tempest,openstack/tempest,varunarya10/tempest,roopali8/tempest,pandeyop/tempest,manasi24/jiocloud-tempest-qatempest,Tesora/tesora-tempest,masayukig/tempest,izadorozhna/tempest,zsoltdudas/lis-tempest,xbezdick/tempest,vedujoshi/tempest,manasi24/jiocloud-tempest-qatempest,JioCloud/tempest,cisco-openstack/tempest,pczerkas/tempest,sebrandon1/tempest,pandeyop/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,openstack/tempest,rakeshmi/tempest,bigswitch/tempest,izadorozhna/tempest,bigswitch/tempest,flyingfish007/tempest