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 <thulith.mallawa@uqconnect.edu.au>