chore: update the link of DCO in PR template (#3944)

chore: update the link of DCO

Signed-off-by: chansuke <moonset20@gmail.com>

.github/pull_request_template.md

@@ -2,7 +2,7 @@ Checklist:
 
 * [ ] Either (a) I've created an [enhancement proposal](https://github.com/argoproj/argo-rollouts/issues/new/choose) 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](https://www.conventionalcommits.org/en/v1.0.0/) with a list of types and scopes found [here](https://github.com/argoproj/argo-rollouts/blob/master/.github/workflows/pr-title-check.yml), (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](https://github.com/argoproj/argoproj)
+* [ ] I've signed my commits with [DCO](https://github.com/argoproj/argoproj/blob/main/community/CONTRIBUTING.md#legal)
 * [ ] 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](https://github.com/argoproj/argo-rollouts/blob/master/USERS.md).
+* [ ] My organization is added to [USERS.md](https://github.com/argoproj/argo-rollouts/blob/master/USERS.md).