[futures.state] Turn note into example

Fixes ISO/CS comment (C++23 proof)

Author: jensmaurer

source/threads.tex

@@ -10259,9 +10259,9 @@
 state.
 The result of a shared state is set by
 respective functions on the asynchronous provider.
-\begin{note}
-Such as promises or tasks.
-\end{note}
+\begin{example}
+Examples for asynchronous providers include promises or tasks.
+\end{example}
 The means of setting the result of a shared state is specified
 in the description of those classes and functions that create such a state object.