Fix comments for nd_item interface.

This is an editorial change.

Author: bader

adoc/chapters/programming_interface.adoc

@@ -11735,7 +11735,7 @@ The additional common special member functions and common member functions are
 listed in <<sec:byval-semantics>> in <<table.specialmembers.common.byval>> and
 <<table.hiddenfriends.common.byval>> respectively.
 
-% interface for nd_item class
+// interface for nd_item class
 [source,,linenums]
 ----
 include::{header_dir}/nditem.h[lines=4..-1]