Fix broken link in Histograms documentation.

Change-Id: Ie1fb9b26d0c4b5e4d767033cf50143e3b4612a3f
Reviewed-on: https://chromium-review.googlesource.com/1134422Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Caitlin Sadowski <supertri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574394}

tools/metrics/histograms/README.md

@@ -264,7 +264,7 @@ The code to record it becomes dead code, and should be removed from the
 codebase along with marking the histogram definition as obsolete. However, if
 histogram would remain useful, the expiration should be extended accordingly
 before it becomes expired. If histogram you care about already expired, see
-[Expired Histogram Whitelist](###Expired histogram whitelist).
+[Expired Histogram Whitelist](#Expired-histogram-whitelist).
 
 For all the new histograms the use of expiry attribute will be strongly
 encouraged and enforced by Chrome metrics team through reviews.