Remove extra semi colon from internal_repo_rocksdb/repo/monitoring/histogram.cc

Summary:
`-Wextra-semi` or `-Wextra-semi-stmt`

If the code compiles, this is safe to land.

Reviewed By: dmm-fb

Differential Revision: D52969001

fbshipit-source-id: d628fa6c5e5d01657fcb7aff7b05dea704ed2025

monitoring/histogram.cc

@@ -69,7 +69,7 @@ void HistogramStat::Clear() {
   for (unsigned int b = 0; b < num_buckets_; b++) {
     buckets_[b].store(0, std::memory_order_relaxed);
   }
-};
+}
 
 bool HistogramStat::Empty() const { return num() == 0; }