Update APIReference-Store.md
Update APIReference-Store.md

Summary: update is a method on `Store`
Closes https://github.com/facebook/relay/pull/518

Reviewed By: kassens

Differential Revision: D2585191

fb-gh-sync-id: a3abb89929610f7b2e0268caeaabdc27f595fb51

voideanvalue/relay,zpao/relay,wincent/relay,wincent/relay,wincent/relay,facebook/relay,kassens/relay,yungsters/relay,chirag04/relay,cesarandreu/relay,xuorig/relay,cpojer/relay,facebook/relay,chentsulin/relay,michaelchum/relay,josephsavona/relay,cesarandreu/relay,benjaminogles/relay,kassens/relay,apalm/relay,mroch/relay,zpao/relay,facebook/relay,andimarek/generic-relay,josephsavona/relay,forter/generic-relay,gabelevi/relay,wincent/relay,iamchenxin/relay,freiksenet/relay,Aweary/relay,atxwebs/relay,cesarandreu/relay,NevilleS/relay,apalm/relay,voideanvalue/relay,andimarek/generic-relay,Aweary/relay,dbslone/relay,venepe/relay,NevilleS/relay,mroch/relay,yungsters/relay,benjaminogles/relay,cpojer/relay,michaelchum/relay,iamchenxin/relay,gabelevi/relay,gabelevi/relay,zpao/relay,josephsavona/relay,kassens/relay,facebook/relay,chentsulin/relay,andimarek/generic-relay,freiksenet/relay,voideanvalue/relay,xuorig/relay,yungsters/relay,heracek/relay,benjaminogles/relay,atxwebs/relay,Aweary/relay,atxwebs/relay,wincent/relay,NevilleS/relay,facebook/relay,venepe/relay,venepe/relay,chirag04/relay,wincent/relay,iamchenxin/relay,chentsulin/relay,chirag04/relay,iamchenxin/relay,rosskevin/relay,voideanvalue/relay,xuorig/relay,xuorig/relay,dbslone/relay,kassens/relay,xuorig/relay,voideanvalue/relay,voideanvalue/relay,kassens/relay,apalm/relay,cpojer/relay,kassens/relay,rosskevin/relay,facebook/relay,xuorig/relay,atxwebs/relay,mroch/relay,cpojer/relay,heracek/relay,michaelchum/relay,heracek/relay,freiksenet/relay