[etherpad-lite]: Fix update instructions

[bugtrapper]

Since the 2.x version, updating an Etherpad installation via git pull often results in a broken installation and requires a fresh install.

[bugtrapper]

The Etherpad developers suggest using

git checkout <version>
git switch -c <version>

instead of

git checkout  -- src/package.json
git pull origin <version>

I don't know yet if that solves the problem or even works reliably on uberspace.