Typo in promp in act_lsys_4

[BergProfJ]

Describe the bug
The instructions for act_lsys_4 (Sec 12.18.1) read "Rather than use the inst string supplied here, use the code from the string chapter, and write your own applyRules function to implement this L-system. "

Instead of the "string chapter," I believe we mean to refer to the "string examples above."

To Reproduce
Steps to reproduce the behavior:

1. Go to https://runestone.academy/runestone/static/heidelberg_ac101f18/Functions/l_systems.html
2. Scroll down to prompt for 'ActiveCode:8 (act_lsys_4)'
3. See error in prompt

Expected behavior
Instead of the wording "string chapter," I believe we mean to refer to the "string examples above."

Screenshots

Desktop (please complete the following information):

• OS: MacOS
• Browser Chrome
• Version 70.0.3538.110

[dichharai]

PR #185