[futures.async] Use code font for "std::async"

Author: K-ballo

source/threads.tex

@@ -4762,7 +4762,7 @@
 \returns An object of type
 \tcode{future<result_of_t<decay_t<F>(decay_t<Args>...)>{>}} that refers
 to the shared state created by this call to \tcode{async}.
-\enternote If a future obtained from std::async is moved outside the local scope,
+\enternote If a future obtained from \tcode{std::async} is moved outside the local scope,
 other code that uses the future must be aware that the future's destructor may
 block for the shared state to become ready. \exitnote