Correct width and height documentation

The constraint:set_width function listed ‘height’ as a required parameter; the constraint:set_height function listed ‘width’ as a required parameter. Swapped those around.

lib/wibox/container/constraint.lua

@@ -99,8 +99,8 @@ end
 
 --- Set the maximum width to val. nil for no width limit.
 --
--- @property height
--- @tparam number height
+-- @property width
+-- @tparam number width
 -- @propemits true false
 
 function constraint:set_width(val)
@@ -115,8 +115,8 @@ end
 
 --- Set the maximum height to val. nil for no height limit.
 --
--- @property width
--- @tparam number width
+-- @property height
+-- @tparam number height
 -- @propemits true false
 
 function constraint:set_height(val)