Update README.md

[Mistobaan]

seems spellcheck is installed under spellchecker-cli (had to change like this to work)

As part of submitting a pull request, please:

• Attempt to add tests for your change. If you're having difficulty, feel free to open the PR and ask for help.
• If you're adding or changing a command-line flag, update the flags list in the Usage section of the README.
• If you're adding a feature or changing behaviour, document the change in the README.
• Add a line to the Unreleased section of CHANGELOG.md describing your change (and crediting yourself if you'd like!)

[tbroadley]

Right you are, thank you!