bugfix: Fix swapped instance and general class names on X11

This let statement swapped the two names, resulting in incorrect
behavior since commit d7ec899d. That commit did not actually introduce
the swap, but the previous code swapped it again before setting the
WM_CLASS property, so no issue was ever observed.

It also brings the documentation in line with the implementation since the
parent commit, and with the ICCCM standard, which states the following
about the WM_CLASS property [1]:

  The two strings, respectively, are:
  * A string that names the particular instance of the application [...]
  * A string that names the general class of applications [...]

[1] https://www.x.org/releases/current/doc/xorg-docs/icccm/icccm.html#WM_CLASS_Property

src/platform_impl/linux/x11/window.rs

@@ -408,7 +408,7 @@ impl UnownedWindow {
 
             // WM_CLASS must be set *before* mapping the window, as per ICCCM!
             {
-                let (class, instance) = if let Some(name) = window_attrs.platform_specific.name {
+                let (instance, class) = if let Some(name) = window_attrs.platform_specific.name {
                     (name.instance, name.general)
                 } else {
                     let class = env::args_os()