fixup: <cname> header

Author: jensmaurer

source/support.tex

@@ -5876,7 +5876,7 @@
 \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{cname}
+otherwise the corresponding header of the form \libheader{c\placeholder{name}}
 provides the same facilities and
 assuredly defines them in namespace \tcode{std}.
 \end{note}