docs: improve deb fields var expansion example

[scop]

$CI_REPOSITORY_URL is not quite appropriate for Vcs-Git, use $CI_PROJECT_URL with Vcs-Browser instead.

• https://docs.gitlab.com/ee/ci/variables/predefined_variables.html
• https://www.debian.org/doc/debian-policy/ch-controlfields.html#version-control-system-vcs-fields

[codecov[bot]]

Codecov Report

Merging #693 (8e99b6c) into main (562bc43) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #693   +/-   ##
=======================================
  Coverage   74.80%   74.80%           
=======================================
  Files          10       10           
  Lines        2385     2385           
=======================================
  Hits         1784     1784           
  Misses        425      425           
  Partials      176      176           

[caarlos0]

nice, thank you!