docs: fix typo in $state (#14535)

Author: paoloricciuti

documentation/docs/02-runes/02-$state.md

@@ -215,7 +215,7 @@ let total = add+++(() => a, () => b)+++;
 console.log(+++total()+++); // 3
 
 a = 3;
-a = 4;
+b = 4;
 console.log(+++total()+++); // 7
 ```