Update platforms.py (#4203)

Rewords root user message to include the correct syntax on running as a different user

celery/platforms.py

@@ -86,7 +86,7 @@ ROOT_DISCOURAGED = """\
 You're running the worker with superuser privileges: this is
 absolutely not recommended!
 
-Please specify a different user using the -u option.
+Please specify a different user using the --uid option.
 
 User information: uid={uid} euid={euid} gid={gid} egid={egid}
 """