Update Gitter link in README.md (#1175)

The link "https://gitter.im/juniper-graphql" responds with a 404. It appears
that the link should be updated to "https://gitter.im/juniper-graphql/Lobby".

Co-authored-by: Christian Legnitto <[email protected]>

Author: thirsty-wing

README.md

@@ -5,7 +5,7 @@
 [![Build Status](https://dev.azure.com/graphql-rust/GraphQL%20Rust/_apis/build/status/graphql-rust.juniper)](https://dev.azure.com/graphql-rust/GraphQL%20Rust/_build/latest?definitionId=1)
 [![codecov](https://codecov.io/gh/graphql-rust/juniper/branch/master/graph/badge.svg)](https://codecov.io/gh/graphql-rust/juniper)
 [![Crates.io](https://img.shields.io/crates/v/juniper.svg?maxAge=2592000)](https://crates.io/crates/juniper)
-[![Gitter chat](https://badges.gitter.im/juniper-graphql/gitter.svg)](https://gitter.im/juniper-graphql)
+[![Gitter chat](https://badges.gitter.im/juniper-graphql/gitter.svg)](https://gitter.im/juniper-graphql/Lobby)
 
 ---