Update action checkout to the new default branch

[triat]

Hello,

I recently saw that there is a new default branch on the checkout action and that your documentation is pointing to the wrong one. (as you can see in the picture here)

I propose an update to the new default one.

Another proposition could be to define actions/checkout@v2 as they propose in their "Usage" documentation https://github.com/actions/checkout#usage

I let you decide what you prefer and I'll update the PR if needed.

Thanks