[lex.phases] Add cross-reference to [lex.header] (#7763)

Author: jensmaurer

source/lex.tex

@@ -159,7 +159,7 @@
 characters into preprocessing tokens is context-dependent.
 \begin{example}
 See the handling of \tcode{<} within a \tcode{\#include} preprocessing
-directive\iref{cpp.include}.
+directive\iref{lex.header,cpp.include}.
 \end{example}
 
 \item The source file is analyzed as a \grammarterm{preprocessing-file}\iref{cpp.pre}.