[class.member.lookup] Also change "N" to "M" in note.

This change should have been part of the resolution of CWG2642 (via
CWG Motion 1), but was accidentally omitted there.

Author: tkoeppe

source/basic.tex

@@ -1618,7 +1618,7 @@
 which shall not be an invalid set, are included in the result.
 \begin{note}
 Overload resolution will discard those
-that cannot convert to the type specified by $N$\iref{temp.over}.
+that cannot convert to the type specified by $M$\iref{temp.over}.
 \end{note}
 
 \pnum