Update challenges.md

Author: bowbahdoe

src/variables/challenges.md

@@ -73,6 +73,7 @@ B
 A
 ```
 
+
 ```java,editable
 void main() {
     String a = "A";
@@ -87,6 +88,11 @@ void main() {
 }
 ```
 
+You are not allowed to write `b = "A";` or `a = "B";` 
+
+## Challenge 5
+
+If your solution to the previous challenge was this:
 
 ## Challenge 5