Gatsby: [docs] update path for using a local starter

Description

In the Using a local starter section of the starter docs, the instructions provided will incorrectly try to access a starter at the Code org on GitHub with this message in the console:

info Creating new site from git: https://github.com/Code/blog.git

In order to create a local starter with the command, the path needs to be absolute or relative and the gatsby-cli interprets the command in the docs now as being from a GitHub repo.

What to do

• change the path here instead to ./Code/my-local-starter
• change the path here instead to ./Code/my-local-starter