Printing API: Fix crx file name in comment

Bug: 992889
Change-Id: I1e301e40e838e7c8072f47c2a6bf8202473e0f1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868883Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Nikita Podguzov <nikitapodguzov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707474}

chrome/test/data/extensions/api_test/printing_metrics/update_manifest.xml

 <?xml version='1.0' encoding='UTF-8'?>
 <!--
-  This update manifest points to the enterprise_device_attributes.crx file.
+  This update manifest points to the printing_metrics.crx file.
   "mock.http" is a placeholder that gets substituted with the test server
   address in runtime.
 -->