Fix summary for identifiability study generation metric.

The current summary references a method that doesn't exit.

Bug: 973801
Change-Id: I7a0bbbb2460419dbe24d60f60d8a09af45ae80aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2174647Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781989}

tools/metrics/ukm/ukm.xml

@@ -4924,7 +4924,7 @@ be describing additional metrics about the same event.
       partition the results.
 
       The name suffix (.626) was chosen so that the resulting metric hash as
-      determined by base::DigestForMetrics() has all 8 LSBs set to 0. Such a
+      determined by base::HashMetricName() has all 8 LSBs set to 0. Such a
       metric hash does not collide with any other metric hash generated by
       blink::IdentifiabilityMetricBuilder().
     </summary>