docs: replace relative path

[sho-hata]

If you run the example of trying the wazero CLI via Docker as is, it will fail.

docker: Error response from daemon: create ./testdata/: "./testdata/" includes invalid characters for a local volume name, only "[a-zA-Z0-9][a-zA-Z0-9_.-]" are allowed. If you intended to pass a host directory, use absolute path.
See 'docker run --help'

This is because relative paths cannot be specified for docker volumes.
Specifying the $PWD environment variable allows the reader to try it out smoothly.

[achille-roussel]

Thanks for the fix!