Avast is usually very accurate in these type of detections as sites are being hacked as a very high rate.
Whilst it is possible to exclude sites in the avastUI, Real-time Shields, Web Shield, Expert Settings, Exclusions and enter the full path to the site.
However, I would suggest not doing that until you have investigate it fully.
What exactly is the problem, e.g. is avast alerting (if so what is the full info or post an image of the alert) ?
What is the URL that is causing the problem (change http or www to hXXp, wXw so it isn't an active link) ?
