Parameter examples generate incorrect OpenAPI schema

```rust
#[derive(Serialize, FromForm, JsonSchema)]
#[schemars(example = "example_param")]
struct Param(i32);

fn example_param() -> Param {
Param(10)
}

#[openapi]
#[get("/example?")]
fn get_param(param: Param) {}
```
Returns a spec containing
```json
"examples": [
10
]
```
instead of
```json
"example": 10
```

Using rocket-okapi 0.8.0-rc.2