Update prob_dist.md

Author: longye-tian

lectures/prob_dist.md

@@ -884,7 +884,7 @@ x_costco = prices.pct_change()[1:] * 100
 fig, ax = plt.subplots()
 ax.violinplot([x_amazon, x_costco])
 ax.set_ylabel('monthly return (percent change)')
-ax.set_xlabel('Retailer')
+ax.set_xlabel('retailers')
 
 ax.set_xticks([1, 2])
 ax.set_xticklabels(['Amazon', 'Costco'])