Correct typo in setting upload base ref

Author: rneatherway

lib/upload-lib.js

@@ -258,7 +258,7 @@ export function buildPayload(
         const githubEvent = JSON.parse(
           fs.readFileSync(process.env.GITHUB_EVENT_PATH, "utf8")
         );
-        payloadObj.base_ref = `refs/heads/$githubEvent.pull_request.base.ref`;
+        payloadObj.base_ref = githubEvent.pull_request.base.ref;
         payloadObj.base_sha = githubEvent.pull_request.base.sha;
       }
     }