Update tutorials/bvp-example.md

[abhro]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• Any new documentation only uses public API

Additional context

Use explicit function names (the p_1 and p_2 weren't referenced anywhere else) and break up a relatively long line of code.