Fix typo in a comment

gitchart.py

@@ -137,7 +137,7 @@ class GitChart(object):
             sorted_keys = sorted_keys[-1 * max_keys:]
         bar_chart.x_labels = sorted_keys[:]
         if max_x_labels > 0 and len(bar_chart.x_labels) > max_x_labels:
-            # reduce number of x labels for readibility: keep only one label
+            # reduce number of x labels for readability: keep only one label
             # on N, starting from the end
             num = max(2, (len(bar_chart.x_labels) // max_x_labels) * 2)
             count = 0