fix: corrects example usage of accessibility prop with custom announcements

[thawkin3]

In this example page for writing your own custom screen reader announcements, the docs are missing a set of curly braces. The specification for the accessibility prop is that it is an object which can have an announcements key. That would require you to write code like this:

accessibility={{
  announcements,
}}

Instead, the docs show only one set of curly braces, indicating that the accessibility prop is just an object with properties like onDragStart, onDragMove, onDragOver, onDragEnd, and onDragCancel, which is incorrect.

accessibility={
  announcements,
}

Hope this helps!

[thawkin3]

Looks like this PR already had me beat: #14. This contains the same fix. If we could merge his earlier PR, that'd be great!