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

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/*

Start at the linked “set up your development environment” tutorial page and inspect the documented `multipass mount --type native` command. Update the mount destination so it produces `~/fastapi-demo` rather than `~/~/fastapi-demo`, then verify the command shown matches the reported working form.

Written by the indexing model from the issue text.