correct query => mutation in MutationRequest.md
correct query => mutation in MutationRequest.md

Summary: Closes https://github.com/facebook/relay/pull/964

Reviewed By: yungsters

Differential Revision: D3069970

Pulled By: kassens

fb-gh-sync-id: b8b09648f9973f91b2e8f3afa23c16af19012c89
shipit-source-id: b8b09648f9973f91b2e8f3afa23c16af19012c89

atxwebs/relay,facebook/relay,benjaminogles/relay,zpao/relay,facebook/relay,iamchenxin/relay,facebook/relay,atxwebs/relay,xuorig/relay,NevilleS/relay,kassens/relay,chentsulin/relay,xuorig/relay,gabelevi/relay,voideanvalue/relay,cpojer/relay,apalm/relay,kassens/relay,xuorig/relay,chentsulin/relay,josephsavona/relay,iamchenxin/relay,wincent/relay,gabelevi/relay,zpao/relay,NevilleS/relay,voideanvalue/relay,dbslone/relay,mroch/relay,josephsavona/relay,atxwebs/relay,cpojer/relay,iamchenxin/relay,dbslone/relay,wincent/relay,benjaminogles/relay,wincent/relay,rosskevin/relay,apalm/relay,NevilleS/relay,cesarandreu/relay,yungsters/relay,kassens/relay,xuorig/relay,freiksenet/relay,cpojer/relay,chentsulin/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,kassens/relay,xuorig/relay,voideanvalue/relay,gabelevi/relay,cesarandreu/relay,facebook/relay,facebook/relay,chirag04/relay,iamchenxin/relay,freiksenet/relay,mroch/relay,voideanvalue/relay,apalm/relay,atxwebs/relay,wincent/relay,zpao/relay,yungsters/relay,facebook/relay,cesarandreu/relay,freiksenet/relay,rosskevin/relay,mroch/relay,chirag04/relay,yungsters/relay,kassens/relay,wincent/relay,benjaminogles/relay,kassens/relay,chirag04/relay,wincent/relay,josephsavona/relay,cpojer/relay