[1808] unrelated doc fix: updated the dsescription of find().

It seemed to be based on an older version of implementation, mentioning
the now-nonexistent Zone class.  Updated the description to reflect the
current status of code.

src/lib/datasrc/memory_datasrc.h

@@ -65,9 +65,7 @@ public:
     /// \brief Returns the class of the zone.
     virtual isc::dns::RRClass getClass() const;
 
-    /// \brief Looks up an RRset in the zone.
-    ///
-    /// See documentation in \c Zone.
+    /// \brief Find an RRset in the datasource
     virtual ZoneFinderContextPtr find(const isc::dns::Name& name,
                                       const isc::dns::RRType& type,
                                       const FindOptions options =