fix missing quote

Author: fritzmg

components/mailer.rst

@@ -196,7 +196,7 @@ local_domain
 
 The domain name to use in ``HELO`` command.
 
-    $dsn = 'smtps://smtp.example.com?local_domain=example.org
+    $dsn = 'smtps://smtp.example.com?local_domain=example.org'
 
 restart_threshold
 ~~~~~~~~~~~~~~~~~
@@ -205,14 +205,14 @@ The maximum number of messages to send before re-starting the transport. Can be
 used together with ``restart_threshold_sleep`` which is the number of seconds to 
 sleep between stopping and re-starting the transport.
 
-    $dsn = 'smtps://smtp.example.com?restart_threshold=10&restart_threshold_sleep=1
+    $dsn = 'smtps://smtp.example.com?restart_threshold=10&restart_threshold_sleep=1'
 
 ping_threshold
 ~~~~~~~~~~~~~~
 
 The minimum number of seconds between two messages required to ping the server.
 
-    $dsn = 'smtps://smtp.example.com?ping_threshold=200
+    $dsn = 'smtps://smtp.example.com?ping_threshold=200'
 
 Sending emails asynchronously
 -----------------------------