Update solved.json

The existing solution returns `fn is not a function` in the repl console. By updating this it fixes it!

langs/en/tutorials/reactivity_on/solved.json

@@ -2,7 +2,7 @@
   "files": [
     {
       "name": "main",
-      "content": "import { render } from \"solid-js/web\";\nimport { createSignal, createEffect, on } from \"solid-js\";\n\nconst App = () => {\n  const [a, setA] = createSignal(1);\n  const [b, setB] = createSignal(1);\n\n  createEffect(on(a, (a) => {\n    console.log(a, b());\n  }, { defer: true }));\n\n  return <>\n    <button onClick={() => setA(a() + 1)}>Increment A</button>\n    <button onClick={() => setB(b() + 1)}>Increment B</button>\n  </>\n};\n\nrender(App, document.getElementById(\"app\"));\n"
+      "content": "import { render } from \"solid-js/web\";\nimport { createSignal, createEffect, on } from \"solid-js\";\n\nconst App = () => {\n  const [a, setA] = createSignal(1);\n  const [b, setB] = createSignal(1);\n\n  createEffect(on(a, (a) => {\n    console.log(a, b());\n  }, { defer: true }));\n\n  return <>\n    <button onClick={() => setA(a => a + 1)}>Increment A</button>\n    <button onClick={() => setB(b => b + 1)}>Increment B</button>\n  </>\n};\n\nrender(App, document.getElementById(\"app\"));\n"
     }
   ]
 }