Update 14-looping-data-sets.md

This avoids the FutureWarning: The default value of numeric_only in DataFrame.mean is deprecated. In a future version, it will default to False. In addition, specifying 'numeric_only=None' is deprecated. Select only valid columns or specify the value of numeric_only to silence this warning.

episodes/14-looping-data-sets.md

@@ -201,7 +201,7 @@ for filename in glob.glob('data/gapminder_gdp*.csv'):
     # retrieve the last string in the list that split returns (`<region>.csv`), 
     # and then remove the `.csv` extension from that string.
     region = filename.split('_')[-1][:-4] 
-    dataframe.mean().plot(ax=ax, label=region)
+    dataframe.mean(numeric_only=True).plot(ax=ax, label=region)
 plt.legend()
 plt.show()
 ```