docs: fix charm name in httpbin deploy command

[dwilding]

Our httpbin demo charm packs to a file called httpbin-demo_amd64.charm not httpbin-demo_ubuntu-22.04-amd64.charm (depending on the architecture). That's because the charm's charmcraft.yaml has

base: ubuntu@24.04
platforms:
  amd64:
  arm64:

instead of

bases:
  - build-on:
    - name: ubuntu
      channel: "22.04"
    run-on:
    - name: ubuntu
      channel: "22.04"

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