doc: getting_started: linux: fixup dnf usage
doc: getting_started: linux: fixup dnf usage

Fixup a command where dnf should be invoked with sudo.
If this command is ran without sudo, dnf will return:

`Error: This command has to be run with superuser
privileges (under the root user on most systems).`

Signed-off-by: Wilfred Mallawa <e9141b0d3b8bc048c4be34b9bda1d9c74f7e7671@uqconnect.edu.au>

galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr