correct another typo

README.translators

@@ -26,7 +26,7 @@ the following rules:
 
     "XXXt" means "XXX translated strings".
     "YYYf" means "YYY fuzzy strings strings".
-    "ZZZt" means "ZZZ translated strings".
+    "ZZZu" means "ZZZ untranslated strings".
     YYY or ZZZ may be omitted if they are null.
 
     This file contents MUST be encoded in UTF-8.