Update ActivityTraceFlags.xml (#2918)

Author: Ron Petrusha

xml/System.Diagnostics/ActivityTraceFlags.xml

@@ -22,7 +22,7 @@
     <remarks>
       <format type="text/markdown"><![CDATA[  
 
-An `ActivityTraceFflags` value is returned by the <xref:System.Diagnostics.Activity.ActivityTraceFlags?displayProperty=nameWithtype> property.
+An `ActivityTraceFlags` value is returned by the <xref:System.Diagnostics.Activity.ActivityTraceFlags?displayProperty=nameWithType> property.
 
      ]]></format>
     </remarks>
@@ -69,4 +69,4 @@ An `ActivityTraceFflags` value is returned by the <xref:System.Diagnostics.Activ
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>