[lex.pptoken] Fix indefinitive article for consistency (#7324)

Author: Andreas-Krug

source/lex.tex

@@ -577,7 +577,7 @@
 except that a \grammarterm{header-name}\iref{lex.header} is only formed
 \begin{itemize}
 \item
-after the \tcode{include} or \tcode{import} preprocessing token in an
+after the \tcode{include} or \tcode{import} preprocessing token in a
 \tcode{\#include}\iref{cpp.include} or
 \tcode{import}\iref{cpp.import} directive, or