Update FeatureEngagement readme

The histogram file has changed and the actions file does not require that exact base::Feature name.

Change-Id: I2151c8e388621f5322ec831e8a0b4d89f1695b75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632648Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845576}

components/feature_engagement/README.md

@@ -239,10 +239,10 @@ To enable UMA tracking, you need to make the following changes to the metrics
 configuration:
 
 1.  Add feature to the histogram suffix `IPHFeatures` in:
-    `//tools/metrics/histograms/histograms.xml`.
+    `//tools/metrics/histograms/histograms_xml/histogram_suffixes_list.xml`.
     *   The suffix must match the `base::Feature` `name` member of your feature.
 1.  Add feature to the actions file at: `//tools/metrics/actions/actions.xml`.
-    *   The suffix must match the `base::Feature` `name` member.
+    *   The suffix must match the `base::Feature` `name` member with `IPH_` stripped.
     *   Find the `<action-suffix>` entry at the end of the file, where the
         following `<affected-action>`s are listed:
         *   `InProductHelp.NotifyEvent.IPH`