Clarify the summary of some prefetch histograms.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index bfa3a812ad132d2b9fcde48a95f66216923b35c4..94e2d5a9cbb4fbed8d31cfac178f1528754d6207 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -27099,8 +27099,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Net.Prefetch.HitBytes" units="bytes">
   <owner>jkarlin@chromium.org</owner>
   <summary>
-    Bytes read for requests that were served from a cache entry filled by a
-    prefetch request. An entry is considered filled by a prefetch if its
+    Bytes read for requests that were served from a cache entry whose
     unused_since_prefetch bit is true.
   </summary>
 </histogram>
@@ -27139,8 +27138,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Net.Prefetch.TimeSpentOnPrefetchHit" units="ms">
   <owner>jkarlin@chromium.org</owner>
   <summary>
-    Time spent on requests that were served from a cache entry filled by a
-    prefetch request. An entry is considered filled by a prefetch if its
+    Time spent on requests that were served from a cache entry whose
     unused_since_prefetch bit is true.
   </summary>
 </histogram>