Improve tutorial example

This tutorial example only handles XCB_EXPOSURE and XCB_KEY_RELEASE
events and ignores everything else. Thus, there is no point in asking
for more kinds of events.

A while ago, I ported this tutorial to x11rb [1]. Recently, I received a
pull request [2] removing these unnecessary event masks. This commit is
thus only partially by me and the 'issue' was originally found by the
author of [2].

[1]: https://github.com/psychon/x11rb/blob/master/x11rb/examples/tutorial.rs
[2]: https://github.com/psychon/x11rb/pull/754

Signed-off-by: Uli Schlachter <psychon@znc.in>

doc/tutorial/index.html

@@ -2297,9 +2297,7 @@ int main ()
   values[0] = screen->white_pixel;
   values[1] =
     XCB_EVENT_MASK_KEY_RELEASE |
-    XCB_EVENT_MASK_BUTTON_PRESS |
-    XCB_EVENT_MASK_EXPOSURE |
-    XCB_EVENT_MASK_POINTER_MOTION;
+    XCB_EVENT_MASK_EXPOSURE;
   cookie_window = xcb_create_window_checked (c,
                                              screen->root_depth,
                                              window, screen->root,