fix typo on ax's attribute name

sohstationviewer/view/plotting/plotting_widget/plotting_axes.py

@@ -222,7 +222,7 @@ class PlottingAxes:
 
         # set samples' total on right side
         if len(sample_no_list) == 1:
-            ax.center_total_points_lbl = ax.text(
+            ax.center_total_point_lbl = ax.text(
                 1.005, 0.5,
                 sample_no_list[0],
                 horizontalalignment='left',