chore: update the link of DCO in PR template

[chansuke]

Update the link in PR template to the CONTRIBUTING.md which has a description of DCO.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.76%. Comparing base (53c4f12) to head (0a210f0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3944      +/-   ##
==========================================
+ Coverage   82.66%   82.76%   +0.09%     
==========================================
  Files         163      163              
  Lines       22895    22895              
==========================================
+ Hits        18927    18948      +21     
+ Misses       3092     3078      -14     
+ Partials      876      869       -7     

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[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 18m 35s ⏱️
113 tests 101 ✅  7 💤 5 ❌
460 runs  425 ✅ 28 💤 7 ❌

For more details on these failures, see this check.

Results for commit 0a210f0.

[github-actions]

Published Unit Test Results

2 280 tests   2 280 ✅  2m 59s ⏱️
  128 suites      0 💤
    1 files        0 ❌

Results for commit 0a210f0.