fix HistogramWindowingImpl copy-{assignment,constructor}

Summary:
their arguments had a typo.
Closes https://github.com/facebook/rocksdb/pull/2821

Differential Revision: D5752408

Pulled By: ajkr

fbshipit-source-id: f2d84489c0c615b12a790f04c42d35cc0dccb02d

monitoring/histogram_windowing.h

@@ -22,8 +22,8 @@ public:
                          uint64_t micros_per_window,
                          uint64_t min_num_per_window);
 
-  HistogramWindowingImpl(const HistogramImpl&) = delete;
-  HistogramWindowingImpl& operator=(const HistogramImpl&) = delete;
+  HistogramWindowingImpl(const HistogramWindowingImpl&) = delete;
+  HistogramWindowingImpl& operator=(const HistogramWindowingImpl&) = delete;
 
   ~HistogramWindowingImpl();