modem-manager: update package prompt name

modem-manager: update package prompt name

Rename the prompt string for consistency with the package directory
name.

Cc: Aleksander Morgado <aleksander@aleksander.es>
Cc: Carlos Santos <casantos@datacom.com.br>
Cc: Petr Vorel <petr.vorel@gmail.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>