Minor typo fix in django.contrib.auth.models.User docs

docs/ref/contrib/auth.txt

@@ -142,7 +142,7 @@ Methods
         .. versionchanged:: 1.6
 
             In Django 1.4 and 1.5, a blank string was unintentionally stored
-            as an unsable password.
+            as an unusable password.
 
     .. method:: check_password(raw_password)