Fix warning in `iron_juniper`
Fix warning in `iron_juniper`

The warning was:

```
warning: unnecessary parentheses around function argument
   --> juniper_iron/src/lib.rs:268:43
    |
268 |             _ => return Ok(Response::with((status::MethodNotAllowed))),
    |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove these parentheses
    |
    = note: #[warn(unused_parens)] on by default
```

graphql-rust/juniper,mhallin/juniper