Tweak contributing guidelines

[AlekSi]

Description

Fix the incorrect sequence of steps.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2886 (81d7fb0) into main (00aeea8) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2886   +/-   ##
=======================================
  Coverage   63.55%   63.55%           
=======================================
  Files         451      451           
  Lines       23512    23512           
=======================================
  Hits        14943    14943           
  Misses       7603     7603           
  Partials      966      966           

Flag	Coverage Δ
integration	56.79% <ø> (ø)
mongodb	4.20% <ø> (ø)
pg	56.71% <ø> (ø)
shard-1	40.94% <ø> (ø)
shard-2	41.58% <ø> (ø)
shard-3	45.08% <ø> (-0.11%)	⬇️
unit	24.34% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[noisersup]

[rumyantseva]

👌