Fix merge workflow (#926)

## Summary
* Fix the import path of the PR description formatter in the merge
workflow, as the path is treated as relative to the repo root and
not the workflow file 

Fixes the issue introduced in

4cd20fa
and observed in
#925 (comment)

.github/workflows/slash-command-merge.yml

@@ -55,7 +55,7 @@ jobs:
               return;
             }
 
-            const formatPR = require('../../tools/ci_format_pr_description.js');
+            const formatPR = require('./tools/ci_format_pr_description.js');
 
             let [ body, body_tail ] = pr_info.body.split(/^---\s*$/m, 2);
             body = formatPR.formatPRdescription(body, 72);