Replace (incorrect) reference to private DEFAULT_SEARCH_PATH constant.

README.md

@@ -18,8 +18,8 @@ TZInfo requires a source of time zone data. There are two options:
 
 By default, TZInfo will attempt to use TZInfo::Data. If TZInfo::Data is not
 available (i.e. if `require 'tzinfo/data'` fails), then TZInfo will search for a
-zoneinfo directory instead (using the search path specified by
-`TZInfo::ZoneinfoDataSource::DEFAULT_SEARCH_PATH`).
+zoneinfo directory instead (by checking the directories specified by
+`TZInfo::DataSources::ZoneinfoDataSource.search_path` in turn).
 
 If no data source can be found, a `TZInfo::DataSourceNotFound` exception will be
 raised when TZInfo is used. Further information is available