buttons: Add missing docstring to the SplitButton.activate() function

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental/buttons.py

@@ -75,6 +75,7 @@ class SplitButton(Gtk.Box):
         self.emit("clicked")
 
     def activate(self, *args) -> None:
+        """Activate the primary button."""
         self._primary.activate()
 
     @GObject.Property(type=Gtk.Button, flags=GObject.ParamFlags.READABLE)