docs: example in README of ipfs-unixfs-importer

[jasny]

• import fs
• remove undefined variables: file, file2, and options
• fix typo causing error "no such file or directory, open './foo/quxx'"

[github-actions[bot]]

:tada: This PR is included in version ipfs-unixfs-importer-v15.1.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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