Refactor message output format (#772)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/svr/task_executor.py

@@ -281,7 +281,7 @@ def main():
             tk_count = 0
         cron_logger.info("Embedding elapsed({}): {}".format(r["name"], timer()-st))
 
-        callback(msg="Finished embedding({})! Start to build index!".format(timer()-st))
+        callback(msg="Finished embedding({:.2f})! Start to build index!".format(timer()-st))
         init_kb(r)
         chunk_count = len(set([c["_id"] for c in cks]))
         st = timer()