chore: update pull_request_template.md

[fedeci]

I made the "briefly describe what you have done in this PR" comment that many people, among which me, forget to delete.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1699 (a0648e3) into master (f5f4fcd) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1699   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          101       101           
  Lines         2005      2005           
  Branches       452       452           
=========================================
  Hits          2005      2005           

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[profnandaa]

Good catch, thanks!