usbkey bash script and README.md contain spelling errors

I tried out VS Code when creating usbkey. It does not have spell check by default (I'm spoiled by JetBrains). There are spelling errors in the readme, and the bash script has spelling errors in both the source code and its console output.

It's a quick fix but I'm leaving it open for anyone who's new to GitHub or open source contributions.

Inspect README.md and the bash script for the reported spelling errors, including text printed to the console. Start by reading both files and running the script's available usage or output path; done means the misspellings in the README, source, and console output are corrected without changing behavior.

Written by the indexing model from the issue text.