Fix typo in tools/metrics/histograms/README.md

The word "relying" was mistyped as "reying".

This CL fixes the typo.

Change-Id: I899e5d543bf3330fa6bbc1c13bc8216290c0b5c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2233147
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776056}

tools/metrics/histograms/README.md

@@ -156,7 +156,7 @@ UmaHistogramEnumeration("NewTabPageAction", action);
 **Note: this method of defining histogram enums is deprecated. Do not use this
 for new enums *in C++*.**
 
-Many legacy enums define a `kCount` sentinel, reying on the compiler to
+Many legacy enums define a `kCount` sentinel, relying on the compiler to
 automatically update it when new entries are added:
 
 ```c++