Update migration guide with correct v3 counter increment example

Author: medz

docs-v2/src/content/docs/migration.mdx

@@ -73,7 +73,7 @@ Use `.value` assignment instead.
 counter.updateValue((v) => v + 1);
 
 // v3
-counter.value = counter.untrackedValue + 1;
+counter.value += 1;
 ```
 
 **Comparator change:**