Docs: Update example from Window signal files_dropped

[thiagola92]

As mentioned in godot-docs/issues/10318, switching to get_window() may be more appropriated as we are talking about a signal that only exist in Window.

Note: I think that this PR is so small that could be done together with another docs update.

Bugsquad edit: closes godotengine/godot-docs#10318

[Mickeon]

This is likely a remnant from 3.x, where Window did not exist and this signal was present in Viewport, so when this was copy-pasted the example did not account for it.

[tetrapod00]

Like I said in the linked issue, seems like a logical change to me, but I'm not an expert at the relevant area.

Note: I think that this PR is so small that could be done together with another docs update.

For typos and style nitpicks in actual engine code, we do try to batch changes into bigger PRs. But for docs changes, as far as I know small atomic commits like this are perfectly fine, as long as they address an actual issue.

[akien-mga]

Thanks!