Fix: add ServiceWorkerFetchEventResult token key/variant.

This is just typo in the last commit.
ServiceWorkerFetchEventResult should be shown as token kye/variant,
or UMA does not show meaningful result.

Previous CL:
https://chromium-review.googlesource.com/c/chromium/src/+/3809203

Bug: 1347319
Change-Id: I4544b1025f5f12bb943d32fd87e92df31306f725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3855829
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039169}

tools/metrics/histograms/metadata/service/histograms.xml

@@ -517,6 +517,8 @@ chromium-metrics-reviews@google.com.
 
     The records are breakdown by FetchEventResult.
   </summary>
+  <token key="ServiceWorkerFetchEventResult"
+      variants="ServiceWorkerFetchEventResult"/>
 </histogram>
 
 <histogram