Fix typo (#3311)

Author: yudomsft

xml/System.Diagnostics.Tracing/EventAttribute.xml

@@ -369,7 +369,7 @@
  You can use standard .NET Framework substitution operators (for example, {1}) in the string). They will be replaced with the string representation of the corresponding part of the event payload.  
   
 ```csharp  
-[Event(1, Message = "Application Falure: {0}", Level = EventLevel.Error, Keywords = Keywords.Diagnostic)]  
+[Event(1, Message = "Application Failure: {0}", Level = EventLevel.Error, Keywords = Keywords.Diagnostic)]  
         public void Failure(string message) { WriteEvent(1, message); }  
   
 ```