doc: Fix example code in documentation of app.add_websocket

[502E532E]

The current example code in the documentation assigns a function to the endpoint: str | None argument of add_websocket. Example code from docs:

def websocket_route():
    ...

app.add_websocket('/', websocket_route)

Function signature:

add_websocket(rule, endpoint=None, view_func=None, **options)

This does not work. I believe the intended way is to assign the function to the view_func parameter and change the corresponding doc entry with this pull request.