rc.lua: Be consistent about how the wibar is created.

The syntax was a leftover from ancient time where the wibar
constructor could not forward the properties to the wibox and
when the wibox didn't take a `widget` argument.

I make this change mostly to be consistent with the documentation
examples.

awesomerc.lua

@@ -179,11 +179,11 @@ screen.connect_signal("request::desktop_decoration", function(s)
 
     -- @DOC_WIBAR@
     -- Create the wibox
-    s.mywibox = awful.wibar({ position = "top", screen = s })
-
+    s.mywibox = awful.wibar {
+        position = "top",
+        screen   = s,
         -- @DOC_SETUP_WIDGETS@
-    -- Add widgets to the wibox
-    s.mywibox.widget = {
+        widget   = {
             layout = wibox.layout.align.horizontal,
             { -- Left widgets
                 layout = wibox.layout.fixed.horizontal,
@@ -200,6 +200,7 @@ screen.connect_signal("request::desktop_decoration", function(s)
                 s.mylayoutbox,
             },
         }
+    }
 end)
 -- }}}