window: Fix docstring formatting on 'set-focus' signal

This comment was added in 93bcca7f02 but missed a
colon so it never actually generated documentation and the nullable annotation
never made it into GObject-introspection language bindings.

This crude regex does not catch any other instances of this mistake. The @
symbol is used to disambiguate signal doc comments from property doc comments,
since property docs usually don't have parameters.

$ pcre2grep -rM '\\* [A-Z][A-Za-z]*:([a-z\-]*):\n +\* @' gtk/

gtk/gtkwindow.c

@@ -1134,7 +1134,7 @@ gtk_window_class_init (GtkWindowClass *klass)
                                                              20, GTK_PARAM_READWRITE));
 
   /**
-   * GtkWindow:set-focus:
+   * GtkWindow::set-focus:
    * @window: the window which received the signal
    * @widget: (nullable): the newly focused widget (or %NULL for no focus)
    *