[Payments] Fix typo in UKM event name.

Bug: 
Change-Id: Ib8444142e35334ae6668bb59f836d16822d4f4bd
Reviewed-on: https://chromium-review.googlesource.com/572050Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487255}

tools/metrics/ukm/ukm.xml

@@ -870,7 +870,7 @@ be describing additional metrics about the same event.
 
 <event name="PaymentRequest.CheckoutEvents">
   <owner>sebsg@chromium.org</owner>
-  <metric name="CompletionsStatus">
+  <metric name="CompletionStatus">
     <summary>
       How the Payment Request ended. Values defined in the CompletionStatus enum
       of components/payments/core/journey_logger.h.