[Docs] add "3" number in the "berry" term explain. (#3277)

* [Docs] add "3" number in the "berry" term explain.

I added 3" number in the "berry" term explain at `getting-startted/install.md`.  
Because v3 development and shipping still running on "berry".  

Thank you for making excellent tool!

* Update install.md

Co-authored-by: Maël Nison <[email protected]>

Author: ryota-murakami

packages/gatsby/content/getting-started/install.md

@@ -47,7 +47,7 @@ cd ~/path/to/project
 yarn set version berry
 ```
 
-> "Berry" is the codename for the Yarn 2 release line. It's also the name of our [repository](https://github.com/yarnpkg/berry)!
+> "Berry" is the codename for all versions of Yarn from 2 onwards: 2.x, 3.x, etc. It's also the name of our [repository](https://github.com/yarnpkg/berry)!
 
 4. Commit new & updated files. See: [Which files should be gitignored?](/getting-started/qa#which-files-should-be-gitignored)