[headers] Strike incorrect quote of subclause heading (#6832)

Author: eisenwave

source/lib-intro.tex

@@ -1313,7 +1313,7 @@
 \end{footnote}
 
 \pnum
-\ref{support.c.headers}, C standard library headers, describes the effects of using
+Subclause \ref{support.c.headers} describes the effects of using
 the \tcode{\placeholder{name}.h} (C header) form in a \Cpp{} program.
 \begin{footnote}
  The