Fix typo "Serview" to "Service" (#1598)

Author: mfalken

docs/index.bs

@@ -203,7 +203,7 @@ spec: rfc7231; urlPrefix: https://tools.ietf.org/html/rfc7231
   </section>
 
   <section>
-    <h3 id="service-worker-timing">Serview Worker Timing</h3>
+    <h3 id="service-worker-timing">Service Worker Timing</h3>
 
     Service workers mark certain points in time that are later exposed by the <a interface lt="PerformanceNavigationTiming">navigation timing</a> API.