fix: Fix link to CONTRIBUTING.md in PR template

[sentrivana]

Description

The link to CONTRIBUTING.md in the PR template doesn't work (see below), updating it.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.52%. Comparing base (bc990db) to head (ea20b1e).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4808      +/-   ##
==========================================
- Coverage   84.55%   84.52%   -0.04%     
==========================================
  Files         158      158              
  Lines       16513    16513              
  Branches     2866     2866              
==========================================
- Hits        13963    13958       -5     
- Misses       1703     1708       +5     
  Partials      847      847              

see 1 file with indirect coverage changes