[exec.async.ops] Fix bad use of \defnadj

Author: Eelis

source/exec.tex

@@ -196,7 +196,7 @@
 A failure completion, also known as an \defnadj{error}{completion},
 has a single result datum.
 \item
-A cancellation completion, also known as a \defnadj{stopped completion},
+A cancellation completion, also known as a \defnadj{stopped}{completion},
 has no result datum.
 \end{itemize}
 An asynchronous operation's \defnadj{async}{result}