[expr.mptr.oper] Index second definition of 'object expression'. (#1669)

Author: Eelis

source/expressions.tex

@@ -3950,7 +3950,7 @@
 
 \pnum
 Abbreviating \term{pm-expression}{}\tcode{.*}\term{cast-expression} as \tcode{E1.*E2}, \tcode{E1}
-is called the \term{object expression}.
+is called the \defn{object expression}.
 If the dynamic type of \tcode{E1} does not
 contain the member to which
 \tcode{E2} refers, the behavior is undefined.