awful.client: fix minor luadoc issue

Signed-off-by: Maarten Maathuis <madman2003@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/client.lua.in

@@ -374,7 +374,7 @@ function master(screen)
     return getmaster(screen)
 end
 
--- Set the client as slave: put it at the end of other windows.
+--- Set the client as slave: put it at the end of other windows.
 -- @param c The window to set as slave.
 function setslave(c)
     local cls = visible(c.screen)