DOC: Fix calculation of bin centers in multi-histogram

As noted in https://discourse.matplotlib.org/t/tiny-error-in-matplotlib-gallery/22978

examples/statistics/multiple_histograms_side_by_side.py

@@ -45,8 +45,8 @@
 
 # The bin_edges are the same for all of the histograms
 bin_edges = np.linspace(hist_range[0], hist_range[1], number_of_bins + 1)
-centers = 0.5 * (bin_edges + np.roll(bin_edges, 1))[:-1]
 heights = np.diff(bin_edges)
+centers = bin_edges[:-1] + heights / 2
 
 # Cycle through and plot each histogram
 fig, ax = plt.subplots()