Add comment about button name on google example

Author: moy

examples/expl_google.py

@@ -8,6 +8,8 @@
 # Fill-in the form
 browser.select_form('form[action="/search"]')
 browser["q"] = "MechanicalSoup"
+# Note: the button name is btnK in the content served to actual
+# browsers, but btnG for bots.
 browser.submit_selected(btnName="btnG")
 
 # Display links