fix: tree/master sourceDirectory

Closes #3096

lib/workers/pr/pr-body.js

@@ -48,7 +48,9 @@ async function getPrBody(config) {
       let fullUrl = sourceUrl;
       if (sourceDirectory) {
         fullUrl =
-          sourceUrl.replace(/\/?$/, '/') + sourceDirectory.replace('^/?/', '');
+          sourceUrl.replace(/\/?$/, '/') +
+          'tree/master/' +
+          sourceDirectory.replace('^/?/', '');
       }
       references.push(`[source](${fullUrl})`);
     }