[cli] update 'README_UDP.md' (#6265)
[cli] update 'README_UDP.md' (#6265)

This commit updates the `README_UDP.md` to use a bullet list for
possible `type` options in `send` command.
abtink/openthread,abtink/openthread,jwhui/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread,openthread/openthread