Docs: Wrong syntax in example @ technical-reference useViewCast

Hi there!
I've just found a non-working snippet, here's a proposed fix too.

- Issue: Wrong syntax in the useViewCast hook code example

Current implementation:

```typescript
const viewSharedCast = useViewCast(sharedCastHash);
// ...
{sharedCastHash && (

View My Share

)}
```

Expected:

```typescript
const { viewCast } = useViewCast();

const viewSharedCast = () => {
if (sharedCastHash) {
viewCast({
hash: sharedCastHash,
close: false
});
}
};

// ...
{sharedCastHash && (

View My Share

)}
```
- Location: [mini-apps/technical-reference/minikit/hooks/useViewCast](https://github.com/base/docs/blob/0e0950e542a4de37959f4699d00ecc338fe6947d/docs/mini-apps/technical-reference/minikit/hooks/useViewCast.mdx)