Update PLUGINS.md (#1302)
Update PLUGINS.md (#1302)

Updating OPA link to point at contrib repo. This is the new location for the plugin.
linkerd/linkerd,BuoyantIO/linkerd,pawelprazak/linkerd,denverwilliams/linkerd,denverwilliams/linkerd,BuoyantIO/linkerd,linkerd/linkerd,pawelprazak/linkerd,pawelprazak/linkerd,denverwilliams/linkerd,linkerd/linkerd,denverwilliams/linkerd