Outdated default branch name `master` used in example responses

[manedev79]

Observed

API response examples contain the outdated default branch name master.
It is still consistent with the Hello-World repo which does have the default branch master.

Expected

API response examples contain the current default branch name main.
The example Hello-World repo should be updated to use mainas default branch to reflect what people are seeing when creating a new repo today.

How to reproduce

curl -X POST 'https://api.github.com/user/repos' --data-raw '{ "name": "REST API Test" }'  
curl 'https://api.github.com/repos/{user}/REST-API-TEST'