Minor bug in documentation

[Nanak Nihal Khalsa]

In https://zokrates.github.io/toolbox/zokrates_js.html, const artifacts = zokratesProvider.compile(source) gave me the following error

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "main.zok:
         --> 1:30
  |
1 | def main(private field a) -> field { return a * a; }
  |                              ^---
  |
  = expected ty_array".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

This was fixed by changing const source = "def main(private field a) -> field { return a * a; }"; to const source = "def main(private field a) -> field: return a * a";