IX Bid Adapter: add UserID documentation for imuid (prebid#4328)

Co-authored-by: Chris Corbo <[email protected]>

Author: ccorbo

dev-docs/bidders/ix.md

@@ -5,7 +5,7 @@ description: Prebid Index Exchange Bidder Adapter
 biddercode: ix
 pbjs: true
 pbs: false
-userIds: identityLink, netId, fabrickId, zeotapIdPlus, uid2, unifiedId, id5Id, lotamePanoramaId, publinkId, hadronId, pubcid
+userIds: identityLink, netId, fabrickId, zeotapIdPlus, uid2, unifiedId, id5Id, lotamePanoramaId, publinkId, hadronId, pubcid, imuid
 pbs_app_supported: true
 schain_supported: true
 coppa_supported: true