Add footnote softening the use of "wrong"

Co-Authored-By: Ken Fogel <[email protected]>

Author: bowbahdoe

src/variables/challenges.md

@@ -90,7 +90,7 @@ void main() {
 
 ## Challenge 5
 
-Some of the variables in this program are named "wrong." Fix them.
+Some of the variables in this program are named "wrong."[^byconvention] Fix them.
 
 ```java,editable
 void main() {
@@ -100,4 +100,6 @@ void main() {
     String FASTRunner = "bolt";
     String slowRunner = "tortoise";
 }
-```
+```
+
+[^byconvention]: By currently prevalent social conventions. None are actually "wrong" from the perspective of Java.