Fixed major bug EXT-7114 (Preferences plain text chat history also modifies IM history)

- Changed old label with new one.

Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/333/

--HG--
branch : product-engine

indra/newview/skins/default/xui/en/panel_preferences_chat.xml

@@ -330,7 +330,7 @@
     <check_box
      enabled="false"
      height="16"
-     label="Enable plain text chat history"
+     label="Enable plain text IM and chat history"
      layout="topleft"
      left_delta="0"
      name="plain_text_chat_history"