Remove the lake +nightly... instruction in project.md

[semorrison]

Remove the lake +nightly... instruction, as it is no longer needed.

[semorrison]

Could someone verify this independently?

[fpvandoorn]

I had a student last week that had Lean 3 installed. lake was not a recognized command on the command line until you are in a repository that specifies a Lean 4 version. So I think this sentence is still helpful for some people.

You could still keep your change and add "if that doesn't work, you probably have an old version of lake. In that case, use ..."

[semorrison]

No, let's keep it for a while then.