window-list: Fix resetting handler ID

This is embarrassing, although destroy() is expected to only run once,
so the bug shouldn't have an effect in practice.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/78

extensions/window-list/windowPicker.js

@@ -255,6 +255,6 @@ class WindowPickerToggle extends St.Button {
     _onDestroy() {
         if (this._overlayKeyId)
             global.display.disconnect(this._overlayKeyId);
-        this._overlayKeyId == 0;
+        this._overlayKeyId = 0;
     }
 });