Set graphql version to ^15.0.0 in yarn command (#3624)

Summary:
Fix #3622

Pull Request resolved: #3624

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

**Static Docs Preview: relay**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D32182865/V2/relay/)|

|**Modified Pages**|

Reviewed By: captbaritone

Differential Revision: D32182865

Pulled By: alunyov

fbshipit-source-id: a5391a489d8c71954629546d0b909b67bc9cfaf9

website/versioned_docs/version-v12.0.0/getting-started/installation-and-setup.md

@@ -52,7 +52,7 @@ module.exports = {
 Relay Modern requires a Babel plugin to convert GraphQL to runtime artifacts:
 
 ```sh
-yarn add --dev babel-plugin-relay graphql
+yarn add --dev babel-plugin-relay graphql@^15.0.0
 ```
 
 Add `"relay"` to the list of plugins your `.babelrc` file: