fix: supports setting a relative path for the API definition URL

[fahchen]

By setting a relative path, we can ensure that it works regardless of where the server is hosted.
For example, the server may be served from a subpath, which is unknown at compile time.

In the example from the module doc, we can set a relative path ./api/openapi for the Swagger UI,
which is equivalent to the absolute path /api/openapi when the server is hosted at the root path.
But if the server is hosted at a subpath like /myapp, which is unknown at compile time,
we can use a relative path instead.

scope "/api" do
  pipe_through :api
  resources "/users", MyAppWeb.UserController, only: [:index, :create, :show]
  get "/openapi", OpenApiSpex.Plug.RenderSpec, :show
end

scope "/" do
  pipe_through :browser

  get "/swaggerui", OpenApiSpex.Plug.SwaggerUI,
    path: "./api/openapi"
end