Update using-strings warning as suggested in code review

Co-authored-by: BethanyG <BethanyG@users.noreply.github.com>

Author: Yrahcaz7

exercises/practice/sublist/.approaches/using-strings/content.md

@@ -1,7 +1,7 @@
 # Using strings
 
 ~~~~exercism/caution
-**This approach does not work, and this document exists to explain that.**
+**This approach does not work (_it will not generalize to all cases_), and this document exists to explain that.**
 Please do not use it in your code.
 ~~~~