support 'ldap:user' variable replacement properly (#10391)

also update `ldap.go` examples with latest
minio-go changes

Fixes #10367

docs/sts/list-objects-with-ldap-user.json

@@ -0,0 +1,14 @@
+{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Effect": "Allow",
+      "Action": [
+        "s3:ListBucket"
+      ],
+      "Resource": [
+        "arn:aws:s3:::${ldap:user}"
+      ]
+    }
+  ]
+}