Update the vignette of interactive exposure by adding `subset` to `define_parameter`

[LittleBeannie]

Following the creation of the ADEXSUM dataset (see issue #119 ), let's update this vignette (https://merck.github.io/metalite.sl/articles/exposure-duration-histogram.html) from

to

define_parameter(name = "expdur",
                   subset = PARAMCD == "TRTDURD",
                   var = "AVAL",
                   label = "Exposure Duration (Days)",
                   vargroup = "EXDURGR")

[wangben718]

high priority