[diff.mods.to.headers] Remove redundant (and wrong) count of C headers.

Also remove italics as this part is not defining the term "C headers".

Author: sdutoit

source/compatibility.tex

@@ -1745,9 +1745,8 @@
 \pnum
 For compatibility with the Standard C library,
 \indextext{library!C standard}%
-the \Cpp standard library provides the 18
-\textit{C headers}
-(\ref{depr.c.headers}), but their use is deprecated in \Cpp.
+the \Cpp standard library provides the C headers enumerated
+in~\ref{depr.c.headers}, but their use is deprecated in \Cpp.
 
 \rSec2[diff.mods.to.definitions]{Modifications to definitions}