[3.0] Improve language string for indexes

Now that we have 2 indexes that need to be created before use, make the message more generic so it looks right.

Author: jdarwood007

Languages/en_US/Search.php

@@ -83,7 +83,7 @@
 $txt['search_index_remove'] = 'Remove';
 $txt['search_index_resume'] = 'Resume';
 // This string is used in a javascript confirmation popup; don't use entities.
-$txt['search_index_custom_warning'] = 'In order to be able to use a custom index search, you must first create a custom index.';
+$txt['search_index_custom_warning'] = 'In order to be able to use this search index, you must first create it.';
 
 $txt['search_force_index'] = 'Force the use of a search index';
 $txt['search_match_words'] = 'Match whole words only';