Update another instance of WIP to 'Draft'

Continue eradicating the word 'WIP' from our API definitions.

app/graphql/mutations/merge_requests/set_wip.rb

@@ -9,7 +9,7 @@ module Mutations
                GraphQL::BOOLEAN_TYPE,
                required: true,
                description: <<~DESC
-                 Whether or not to set the merge request as a WIP.
+                 Whether or not to set the merge request as a draft.
                DESC
 
       def resolve(project_path:, iid:, wip: nil)