[ifndr:over.literal.reserved] Add \grammarterm for literal-operator-id (#9245)

Author: Andreas-Krug

source/ifndr.tex

@@ -644,7 +644,7 @@
 
 \pnum
 Some literal suffix identifiers are
-reserved for future standardization. A declaration whose literal-operator-id uses such a literal
+reserved for future standardization. A declaration whose \grammarterm{literal-operator-id} uses such a literal
 suffix identifier is ill-formed, no diagnostic required.
 
 \pnum