feat: add new constraint to latest challenge

Author: tomalaforge

docs/src/content/docs/challenges/signal/51-function-call-effect.md

@@ -20,3 +20,7 @@ In this second challenge focusing on Signal effects, we've introduced an input s
 Ideally, the system should log an action only when one is specifically selected. However, we currently face an issue where changing the user also triggers a log entry, even though we do not explicitly monitor user changes.
 
 The objective of this challenge is to identify and resolve the cause of these extra triggers. We aim to ensure that logging only occurs when an action is selected.
+
+## Constraints
+
+- You cannot modify the `UserService` file.