docs: fastapi-demo moint point creates extra '~' in a mount path

[amorskoy]

Command multipass mount --type native ~/k8s-tutorial juju-sandbox-k8s:~/fastapi-demo mounts to ~/~/fastapi-demo instead of expected ~/fastapi-demo.

What worked for me:
multipass mount --type native ~/k8s-tutorial juju-sandbox-k8s:fastapi-demo

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Reported from: https://canonical.com/juju/docs/ops/latest/tutorial/from-zero-to-hero-write-your-first-kubernetes-charm/set-up-your-development-environment/