Set graphql version to ^15.0.0 in install commands (#3623)

Summary:
Fix #3622

Pull Request resolved: #3623

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

**Static Docs Preview: relay**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D32182841/V2/relay/)|

|**Modified Pages**|

Reviewed By: captbaritone

Differential Revision: D32182841

Pulled By: alunyov

fbshipit-source-id: 6887c42afddc7fd2a349fd703967cc51adf511a1

website/versioned_docs/version-v12.0.0/getting-started/step-by-step-guide.md

@@ -157,11 +157,11 @@ First we'll add the necessary packages. Note that Relay is comprised of three ke
 ```bash
 # NPM Users
 npm install --save relay-runtime react-relay
-npm install --save-dev relay-compiler graphql babel-plugin-relay
+npm install --save-dev relay-compiler graphql@^15.0.0 babel-plugin-relay
 
 # Yarn Users
 yarn add relay-runtime react-relay
-yarn add --dev relay-compiler graphql babel-plugin-relay
+yarn add --dev relay-compiler graphql@^15.0.0 babel-plugin-relay
 ```
 
 ### 4.1 Configure Relay Compiler