Add "the" before gem references in owner notification emails

[marcoroth]

While receiving notifications about gem ownership, I noticed the wording felt a bit unnatural without the definite article "the" before references to gems. This small pull request improves readability by adding "the" where appropriate in the owner-related email notifications.

Just a small grammar improvement that, I think, makes the notifications sound more natural. Feel free to close this pull request if you disagree 🙈

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.33%. Comparing base (46ff0a3) to head (8707cbe).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5520      +/-   ##
==========================================
- Coverage   97.11%   94.33%   -2.79%     
==========================================
  Files         454      454              
  Lines        9441     9500      +59     
==========================================
- Hits         9169     8962     -207     
- Misses        272      538     +266     

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