Added missing string.

languages/en.ini

@@ -288,6 +288,7 @@ edit_list_fail = "Sorry, you are not permitted to edit this list"
 edit_list_success = "List successfully updated."
 Edition = "Edition"
 eds_expander_fulltext = "Also search within the full text of the articles"
+eds_expander_relatedsubjects = "Apply equivalent subjects"
 eds_expander_thesaurus = "Apply related words"
 eds_limiter_FC = "Catalog Only"
 eds_limiter_FC1 = "Institutional Repository Only"