build: extend sample-lnd.conf with the new etcd namespace

sample-lnd.conf

@@ -965,6 +965,9 @@ litecoin.node=ltcd
 ; Password for the database user.
 ; db.etcd.pass=longandsekrit
 
+; Etcd namespace to use.
+; db.etcd.namespace=lnd
+
 ; Path to the TLS certificate for etcd RPC.
 ; db.etcd.cert_file=/key/path