fix: show correct weekday in exams

[limwa]

This PR changes the weekday selection logic to show the correct weekday for an exam.

The internationalization library we are using returns a list of weekdays from Sunday through Saturday. The weekday property on DateTimes returns a number from 1 (Monday) through 7 (Sunday). This means that weekday % 7 must be used instead of weekday - 1.

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

Merging #1012 (f200a5a) into master (6ef9a17) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1012   +/-   ##
======================================
  Coverage      16%     16%           
======================================
  Files         201     201           
  Lines        6361    6361           
======================================
  Hits          986     986           
  Misses       5375    5375           

[LuisDuarte1]

good catch 🚀