site/plugins.md: clarify that node ids are different
site/plugins.md: clarify that node ids are different

alban/scope,alban/scope,weaveworks/scope,alban/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,alban/scope,alban/scope,weaveworks/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,alban/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,kinvolk/scope