test/config: fix password format in etcd example

The instance config schema was changed in commit 4bb1eb0e ("config:
remove hashes from credentials.password"), but an example of a config
for etcd was not updated.

The example is tested on Tarantool EE, so we should update it to fix the
testing failure.

Part of #8967

NO_DOC=It is a fix of the testing problem.
NO_CHANGELOG=see NO_DOC
NO_TEST=It is a fix of a test in fact.

doc/examples/config/etcd/remote.yaml

 credentials:
   users:
     replicator:
-      password:
-        plain: 'topsecret'
+      password: 'topsecret'
       roles: [replication]
     client:
-      password:
-        plain: 'secret'
+      password: 'secret'
       roles: [super]
 
 groups: