Fix openapi serialized twice when Rapidoc is served with Rocket

[Rauks]

Hello,

When Rocket serves a Rapidoc built using with_openapi, the served openapi.json is the result of an ApiDoc serialized twice.

rocket::build()
    .mount(
        "/",
        RapiDoc::with_openapi("/api-docs/openapi.json", ApiDoc::openapi()).path("/rapidoc"),
    )

As a result, Rapidoc fails to load the spec:

Inspecting the request response for /api-docs/openapi.json shows a "string of a json":

This PR is a fix proposal that resolves the double serialization.
After the fix, the spec now loads correctly and the /api-docs/openapi.json request response inspection shows "just a json":

[juhaku]

Thanks for the catch 👍