fix typo in session-settings

--- src/pantheon-session/xsessions/meson.build~ 2017-12-26 01:18:00.411839705 +0900
+++ src/pantheon-session/xsessions/meson.build  2017-12-26 01:38:29.090538935 +0900
@@ -1,4 +1,4 @@
 install_data(
     'pantheon.desktop',
-    install_dir: join_paths(get_option('datadir'), 'xessions')
+    install_dir: join_paths(get_option('datadir'), 'xsessions')
 )