Wal-Mart Brightens Dec. Sales Forecast  NEW YORK (Reuters) - Wal-Mart Stores Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WMT.N target=/stocks/quickinfo/fullquote"&gt;WMT.N&lt;/A&gt;, the  world's biggest retailer, on Monday brightened its outlook for  December sales at U.S. stores open at least a year, now  expecting sales to increase 3 percent.