Docs: replace the command to install relay/macro (facebook#7262)

Author: soufDev

docusaurus/docs/adding-relay.md

@@ -14,7 +14,7 @@ npm install --save babel-plugin-relay
 Alternatively you may use `yarn`:
 
 ```sh
-yarn upgrade babel-plugin-relay
+yarn add babel-plugin-relay
 ```
 
 Then, wherever you use the `graphql` template tag, import the macro: