Fix doc for adding sudo privileges to stack user

Writing NOPASSWD directive into /etc/sudoers was throwing
permission denied errors. This commit writes the directive
to the /etc/sudoers.d/stack file instead.

Closes-Bug: #1981541
Change-Id: If30f01aa5f3a33dda79ff4a6892116511c8e1542
diff --git a/doc/source/guides/multinode-lab.rst b/doc/source/guides/multinode-lab.rst
index 79a76de..658422b 100644
--- a/doc/source/guides/multinode-lab.rst
+++ b/doc/source/guides/multinode-lab.rst
@@ -89,7 +89,7 @@
 
 ::
 
-    echo "stack ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
+    echo "stack ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/stack
 
 From here on use the ``stack`` user. **Logout** and **login** as the
 ``stack`` user.