Add instructions on restarting instance in "https://canonical.com/multipass/docs/add-a-network-to-an-existing-instance"

[jnoh]

Description of documentation issue or suggestion
In How to add a network to an existing instance, the reader is instructed to stop their instance, but there is no instruction to restart their instance.

Proposal to fix issue or enact suggestion
Add a note to multipass start [INSTANCE_NAME].

Additional context
https://canonical.com/multipass/docs/add-a-network-to-an-existing-instance

[giuliazanchi]

Thank you for your suggestion, @jnoh !