Update README

Author: martinrotter

resources/skins/plain/README

@@ -6,7 +6,7 @@ Here is overview of all placeholders used throughtout the skin:
     %data% - is replaced by current absolute path to user data folder.
 
   html_wrapper.html:
-    %style% - Skin-wide CSS, will be replaced by custom CSS if skin provides it in file "style.css".
+    %style% - Skin-wide CSS, will be replaced by custom CSS if skin provides it in file "html_style.css".
     %1 - Title of the article.
     %2 - Body of the preview which is composed from:
             - single html_single_message.html (single article preview),
@@ -33,4 +33,4 @@ Here is overview of all placeholders used throughtout the skin:
   html_enclosure_image.html:
     %1 - URL of the attached picture file.
     %2 - MIME type of the picture.
-    %3 - height of the picture thumbnail to be displayed which is loaded from application settings.
+    %3 - height of the picture thumbnail to be displayed which is loaded from application settings.