docs: pr title check fix to link to right doc

Linking to https://github.com/filecoin-project/lotus/blob/master/CONTRIBUTING.md#pr-title-conventions instead of README.

.github/workflows/pr-title-check.yml

@@ -26,7 +26,7 @@ jobs:
               await github.rest.pulls.createReview({
                 ...context.repo,
                 pull_number: context.payload.pull_request.number,
-                body: 'Please update the PR title to match https://github.com/filecoin-project/lotus/blob/master/README.md#pr-title-conventions',
+                body: 'Please update the PR title to match https://github.com/filecoin-project/lotus/blob/master/CONTRIBUTING.md#pr-title-conventions',
                 event: 'REQUEST_CHANGES'
               });
               core.setFailed('PR title does not match the required format');