[exec.awaitables] Add missing word (cplusplus#7340)

Author: timsong-cpp

Diff for: source/exec.tex

@@ -2136,7 +2136,7 @@
 }
 \end{codeblock}
 \begin{note}
-of \exposid{env-promise} are used only for the purpose of type computation;
+Specializations of \exposid{env-promise} are used only for the purpose of type computation;
 its members need not be defined.
 \end{note}