Add missing import on HTTPSRedirectMiddleware docs section

[TheOnlyWayUp]

The docs for HTTPSRedirectMiddleware (here) reference a Middleware class in the code snippet

from starlette.applications import Starlette
from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware

routes = ...

middleware = [
    Middleware(HTTPSRedirectMiddleware)
]

app = Starlette(routes=routes, middleware=middleware)

Middleware isn't defined in that context, the required code

from starlette.middleware import Middleware

Is present in all other examples, leading me to believe that this wasn't intentional.

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This pull request adds an import for a class that's referenced in the code, but not imported.

[Kludex]

Thanks @TheOnlyWayUp 🙏