Fix typo in Italian locale, update Romansh locale

[msom]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [x] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

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Description of Changes

Fixes typo (superfluous comma) in Italian locale and adds week/weeks to Romansh locale

[codecov[bot]]

Codecov Report

Merging #1121 (8e82a3a) into master (5f9dfbe) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1121   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2325      2325           
  Branches       449       449           
=========================================
  Hits          2325      2325           

Impacted Files	Coverage Δ
arrow/locales.py	100.00% <ø> (ø)

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