Fix tls verificaon docs

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

doc/admin.md

@@ -28,7 +28,7 @@ You can use the php-mail function to send mails. This is needed for some webhost
 ### Disable TLS verification for IMAP/SMTP
 Turn off TLS verfication for IMAP/SMTP. This happens globally for all accounts and is only needed in edge cases like with email servers that have a self-signed certificate.
 ```php
-'app.mail.verify-tls-peer' => 'false'
+'app.mail.verify-tls-peer' => false
 ```
 
 ## Troubleshooting