[stmt.expr] Use \grammarterm for expression (#6469)

Author: Eisenwave

source/statements.tex

@@ -188,7 +188,7 @@
 side effects from an expression statement
 are completed before the next statement is executed.
 \indextext{statement!empty}%
-An expression statement with the expression missing is called
+An expression statement with the \grammarterm{expression} missing is called
 a \defnadj{null}{statement}.
 \begin{note}
 Most statements are expression statements --- usually assignments or