docs: Skip export keyword in notification docs (#14633) (#14644)

This change does three things:

1. It removes the `export` keyword. It's not required since the example
   executes a script where the variables are evaluated as an inline
   string. One could even argue that there is a slight security issue
   with using `export` here, since that will expose the credentials to
   all applications started in the current context.
2. It adds a space (` `) before the `PASSWORD` variable. This will keep
   it out of the user's Bash history by default. See [HISTIGNORE][bash].
3. Add a newline for clarity.

[bash]: https://www.gnu.org/software/bash/manual/bash.html#index-HISTIGNORE

Signed-off-by: Andreas Lindhé <andreas@lindhe.io>
Co-authored-by: Andreas Lindhé <lindhe@users.noreply.github.com>

docs/operator-manual/notifications/index.md

@@ -17,8 +17,9 @@ kubectl apply -n argocd -f https://raw.githubusercontent.com/argoproj/argo-cd/st
 * Add Email username and password token to `argocd-notifications-secret` secret
 
 ```bash
-export EMAIL_USER=<your-username>
-export PASSWORD=<your-password>
+EMAIL_USER=<your-username>
+ PASSWORD=<your-password>
+
 kubectl apply -n argocd -f - << EOF
 apiVersion: v1
 kind: Secret