Remove open source checkbox from README

Removed the open source checkbox from the README.

Author: grega

README.md

@@ -3,6 +3,7 @@
 - [ ] Ensure all strings in `src/` are localised
 - [ ] Consider renaming `explain()` to something like `friendlyExplain()`?
 - [ ] Figure out where the built packages should live / be served from
+- [ ] Open source?
 
 A small, runtime-agnostic, library that explains Python error messages in a friendlier way, inspired by [p5.js's Friendly Error System](https://p5js.org/contribute/friendly_error_system/).