docs: make the required backticks in email password more explicit (#23923)

updated the example config to make the needed backticks around the
password more obvious

doc/administration/email-setup.en-us.md

@@ -81,7 +81,7 @@ SMTP_ADDR      = smtp.gmail.com
 SMTP_PORT      = 465
 FROM           = [email protected]
 USER           = example.user
-PASSWD         = ***
+PASSWD         = `***`
 MAILER_TYPE    = smtp
 IS_TLS_ENABLED = true
 ```