Add missing <br> for Ubuntu PPA instructions

[lindhe]

Changes

• The instructions for adding the PPA in Ubuntu were missing a newline. This change fixes that.

Context

This is what the "Download for Linux and Unix" page currently looks like:

I don't know how to render a test site to verify that my fix works, so if you can help me with that it would be great. While this is a dead-simple change, it's never fun to make changes blindly.

[dscho]

I don't know how to render a test site to verify that my fix works, so if you can help me with that it would be great.

@lindhe You should be able to deploy this by

• ensuring that https://github.com/lindhe/git-scm.com/settings/pages has its "Build and deployment" source set to GitHub Actions:
  
• ensuring that GitHub Actions are enabled (e.g. by looking at https://github.com/lindhe/git-scm.com/actions and verifying that it does not encourage you to enable them)
• you may, or may not need to create an environment called github-pages (if in doubt, just create it at https://github.com/lindhe/git-scm.com/settings/environments)
• pushing your change to the gh-pages branch (you could use the GitHub CLI via gh api repos/lindhe/git-scm.com/git/refs/heads/gh-pages -f sha=05d94c42008bb6717c38c7dc7dfd1a7cdb9a9816, or manually triggering the deploy workflow at https://github.com/lindhe/git-scm.com/actions/workflows/deploy.yml (making sure that you select the patch-1 branch).

This should deploy the site to https://lindhe.github.io/git-scm.com/

[dscho]

I verified that the deployed site works: https://git-scm.com/downloads/linux looks correct now.