Update index.html

Co-Authored-By: Marcos Cáceres <marcos@marcosc.com>

Author: tommckee1

index.html

@@ -168,7 +168,7 @@ <h2>Extensions to the <code><dfn data-cite="HR-TIME-2#dfn-performance">Performan
         };
 
         partial interface Performance {
-            PerformanceMark mark(DOMString markName, optional PerformanceMarkOptions markOptions);
+            PerformanceMark mark(DOMString markName, optional PerformanceMarkOptions markOptions = {});
             void clearMarks(optional DOMString markName);
             PerformanceMeasure measure(DOMString measureName, optional (DOMString or PerformanceMeasureOptions) startOrMeasureOptions = {}, optional DOMString endMark);
             void clearMeasures(optional DOMString measureName);