Update Modern-RelayEnvironment.md
Update Modern-RelayEnvironment.md

Summary:
Fixes broken link to Relay Cursor Connections Specificiation
Closes https://github.com/facebook/relay/pull/2390

Differential Revision: D7474902

Pulled By: jstejada

fbshipit-source-id: 6aead9ccbdf20dd0c52e6aa80600b5aad459de8e

voideanvalue/relay,facebook/relay,atxwebs/relay,dbslone/relay,wincent/relay,facebook/relay,facebook/relay,freiksenet/relay,yungsters/relay,voideanvalue/relay,josephsavona/relay,xuorig/relay,xuorig/relay,cpojer/relay,kassens/relay,voideanvalue/relay,cpojer/relay,cpojer/relay,kassens/relay,yungsters/relay,kassens/relay,xuorig/relay,wincent/relay,xuorig/relay,freiksenet/relay,zpao/relay,kassens/relay,atxwebs/relay,josephsavona/relay,wincent/relay,wincent/relay,cpojer/relay,iamchenxin/relay,wincent/relay,voideanvalue/relay,wincent/relay,zpao/relay,iamchenxin/relay,facebook/relay,atxwebs/relay,facebook/relay,iamchenxin/relay,yungsters/relay,atxwebs/relay,voideanvalue/relay,dbslone/relay,iamchenxin/relay,josephsavona/relay,xuorig/relay,kassens/relay,zpao/relay,facebook/relay,freiksenet/relay,kassens/relay,voideanvalue/relay,xuorig/relay