Do not ask users to overwrite their /etc/sudoers file

Give instructions to add permissions for stack user without touching
main config.

Closes-Bug: #1680459
Closes-Bug: #1681418
Change-Id: Idd27e684e63c616466de28c07551729a1e091bdd
diff --git a/doc/source/index.rst b/doc/source/index.rst
index c3bac9d..cbd6971 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -63,7 +63,7 @@
 
 ::
 
-    $ sudo tee <<<"stack ALL=(ALL) NOPASSWD: ALL" /etc/sudoers
+    $ echo "stack ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/stack
     $ sudo su - stack
 
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