Update installation instructions to work with main branch

[tonycuadra]

When attempting to follow the installation instructions here:
https://kampfkarren.github.io/selene/cli/installation.html

I ran into an error with this command:
cargo install --git https://github.com/Kampfkarren/selene selene

It looks like cargo install still defaults to using the master branch and errors out if it does not exist. To workaround this, the correct branch name must be specified:

cargo install --branch main --git https://github.com/Kampfkarren/selene selene

[Kampfkarren]

Aw, that's a shame, I didn't think about that when renaming the branch.

[Kampfkarren]

Apparently this is on nightly, which should fix the problem, but for now, yeah this needs to be fixed.

rust-lang/cargo#9133