[depr.c.macros] Fix "macro" singular when referring to two macros

This should have been applied as part of LWG 4036.

Author: jwakely

source/future.tex

@@ -227,7 +227,7 @@
 \rSec1[depr.c.macros]{Deprecated C macros}
 
 \pnum
-The header \libheader{stdalign.h} has the following macro:
+The header \libheader{stdalign.h} has the following macros:
 \indexheader{stdalign.h}%
 \indexlibraryglobal{__alignas_is_defined}%
 \begin{codeblock}