[support.c.headers.general] Do not use \libheader with placeholder.

Author: timsong-cpp

source/support.tex

@@ -5895,7 +5895,7 @@
 \begin{note}
 The C headers either have no effect,
 such as \libheaderref{stdbool.h} and \libheaderref{stdalign.h}, or
-otherwise the corresponding header of the form \libheader{c\placeholder{name}}
+otherwise the corresponding header of the form \tcode{<c\placeholder{name}>}
 provides the same facilities and
 assuredly defines them in namespace \tcode{std}.
 \end{note}