Fixes typo in `examples/pong_xapp.py`

Changed `[RMR_MS_MSG_PAYLOAD]` to `[RMR_MS_PAYLOAD]`

Issue-Id: RIC-974
Signed-off-by: Abhijit Gadgil <gabhijit@iitbombay.org>
Change-Id: I3f2d0a3263d231ded29dff51ad94841557d24a72
1 file changed