docs(email-notif): Document the {{ INSTALL_PREFIX }} placeholder

co-authored-by: giuaig <[email protected]>

README.md

@@ -484,7 +484,7 @@ Put this file in `/bin`:
 * `systemd-email`: Sends email using sendmail(1). This script also features time-out for not spamming Gmail servers and getting my account blocked.
 
 Put this file in `/etc/systemd/system/`:
-* `[email protected]`: A service that can notify you via email when a systemd service fails. Edit the target email address in this file.
+* `[email protected]`: A service that can notify you via email when a systemd service fails. Edit the target email address in this file, and replace or remove `{{ INSTALL_PREFIX }}` according to your installation.
 
 Now edit `[email protected]` and `[email protected]` to call this service failure.
 ```