Reduce adpex for registry server_route_manifest_writes

This change is to relax apdex for Registry on the server_route_manifest_writes custom route.

It was found in analysis by Matt that 99th percentile response times are close to 1 second, and around 1% of the requests exceed that satisfied threshold to make it hard to maintain the Apdex score, so this change to reduce the threshold slightly to reduce the noise.