Update reference.adoc

doc/mp11/reference.adoc

@@ -17,14 +17,23 @@ The contents of the library are in namespace `boost::mp11`.
 :leveloffset: +1
 
 include::integral.adoc[]
+
 include::list.adoc[]
+
 include::utility.adoc[]
+
 include::algorithm.adoc[]
+
 include::set.adoc[]
+
 include::map.adoc[]
+
 include::function.adoc[]
+
 include::bind.adoc[]
+
 include::integer_sequence.adoc[]
+
 include::tuple_for_each.adoc[]
 
 :leveloffset: -1