Fix readme to use [[ per readme

[rasa]

To be consistent with https://github.com/kvz/bash3boilerplate/edit/master/README.md#Scoping

Thanks for contributing to b3bp! As part of your PR, have you:

• [ ] Added an item in CHANGELOG.md with attribution?
• [ ] Added your name to the README.md
• [ ] Linted your code? (make test should do the trick)

If so, great! Feel free to replace this message with a description of your work and hit submit!

[kvz]

thank you! :heart: