Update docs-preview link (#2318)

Following the migration from Jenkins to Buildkite, docs previews are now available at <repo>_bk_<PR>.

More context in elastic/docs#2898

Author: nkammah

.github/workflows/docs-preview-comment.yml

@@ -15,7 +15,7 @@ jobs:
         with:
           script: |
             const pr = context.payload.pull_request;
-            const comment = `Documentation changes preview: https://${context.repo.repo}_${pr.number}.docs-preview.app.elstc.co/diff`;
+            const comment = `Documentation changes preview: https://${context.repo.repo}_bk_${pr.number}.docs-preview.app.elstc.co/diff`;
 
             github.rest.issues.createComment({
               issue_number: context.issue.number,