[Checkbox] Fixed documentation typo

Resolves #2580

PiperOrigin-RevId: 430473797

docs/components/Checkbox.md

@@ -104,7 +104,7 @@ In code:
 checkbox.isChecked = true
 
 // To listen for a checkbox's checked/unchecked state changes
-checkbox.setOnCheckedChangeListener { buttonView, isChecked
+checkbox.setOnCheckedChangeListener { buttonView, isChecked ->
     // Responds to checkbox being checked/unchecked
 }
 ```