[stmt.expr] use grammarterm for expression

source/statements.tex

@@ -189,7 +189,7 @@
 are completed before the next statement is executed.
 \indextext{statement!null}%
 \indextext{statement!empty}%
-An expression statement with the expression missing is called
+An expression statement with the \grammarterm{expression} missing is called
 a \defn{null statement}.
 \begin{note}
 Most statements are expression statements --- usually assignments or