Fix typo in GraphQL-RelaySpecification.md
Fix typo in GraphQL-RelaySpecification.md

Summary:
I'm not quite sure, but it seems like a typo.
Closes https://github.com/facebook/relay/pull/1597

Reviewed By: kassens

Differential Revision: D4860975

Pulled By: sdwilsh

fbshipit-source-id: eda1c1c58bcb6f2dd61743bf0db0c4a8b139eee2

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