docs: correct readonly `<audio>` binding count (six -> seven)

The <audio> section says there are six readonly bindings but lists seven
(duration, buffered, seekable, seeking, ended, readyState, played). All seven
are genuine readonly bindings in packages/svelte/src/compiler/phases/bindings.js
(only currentTime, paused, volume, muted and playbackRate are bidirectional).

documentation/docs/03-template-syntax/12-bind.md

@@ -261,7 +261,7 @@ You can give the `<select>` a default value by adding a `selected` attribute to
 - [`volume`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/volume)
 - [`muted`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/muted)
 
-...and six readonly ones:
+...and seven readonly ones:
 
 - [`duration`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/duration)
 - [`buffered`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/buffered)