Fix docs for ExportedHistogram

Summary:
`ExportedHistogram` wraps `folly::Histogram`. From the folly docs

https://www.internalfb.com/code/fbsource/[0f0c602d3bc63b33d7b8554fa5976ceb612d4e2b]/fbcode/folly/docs/Histogram.md?lines=54-63

and

https://www.internalfb.com/code/fbsource/[d81cb3266ebe564aa44fb290afc21dbc892eaac8]/fbcode/folly/stats/Histogram.h?lines=50-59

The docstring in HHVM for `bucketSize` is incorrect and led to a significant delay in understanding what was wrong.

Reviewed By: mdko

Differential Revision: D68882151

fbshipit-source-id: 04f332e7ed43dad7d0811840f55b0978e194576e

Author: John Oxley

hphp/util/service-data.h

@@ -211,8 +211,9 @@ ExportedTimeSeries* createTimeSeries(
 /*
  * Create a histogram counter named 'name'. Return an existing one if it has
  * already been created.
+ * The number of buckets is (max - min) / bucketSize
  *
- * 'bucketSize' specifies how many buckets to track for the histogram.
+ * 'bucketSize' specifies the size of each bucket
  * 'min' is the minimal value in the histogram.
  * 'max' is the maximal value in the histogram.
  * 'exportPercentile' specifies at what percentile values we should report the