README: Replace `fly` with `${FLY_CMD}`
README: Replace `fly` with `${FLY_CMD}`

In 6b0ef65 and 2e1cc94 we stopped recommending that people download the
`fly` binary to `/usr/local/bin`. Instead `vagrant/deploy.sh` downloads the
binary into your PWD and sets `FLY_CMD`. This command in the README is the
last reference to a `fly` binary on your `PATH`.

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