[syntax] Use a different example (#959)

Author: cpplearner

source/intro.tex

@@ -447,8 +447,8 @@
 delimiters (e.g., \grammarterm{declaration-seq} is a sequence of
 declarations).
 \item \grammarterm{X-list} is one or more \grammarterm{X}'s separated by
-intervening commas (e.g., \grammarterm{expression-list} is a sequence of
-expressions separated by commas).
+intervening commas (e.g., \grammarterm{identifier-list} is a sequence of
+identifiers separated by commas).
 \end{itemize}%
 \indextext{notation!syntax|)}