MAINTAINERS: Fix label for 'Nuvoton_Numicro Platforms'

This commit updates the label name in the 'Nuvoton_Numicro Platforms'
entry to match the actual GitHub label name, which is 'platform:
Nuvoton Numicro Platforms'.

In addition, the entry name is also updated to match the label styling
(underscore is removed).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>