Notification docs: Add SMTP port to gmail configuration (#573)

The notification shows an example of how to get it working for gmail smtp servers, but it's missing the port parameter required to get the SMTP connection working.

Without this setting, you get a "connection refused" connecting to port 25.

Given the popularity of gmail, it would be nice to have this example more accurate.

docs/notifications.md

@@ -47,6 +47,7 @@ docker run -d \
   -e WATCHTOWER_NOTIFICATION_EMAIL_FROM=fromaddress@gmail.com \
   -e WATCHTOWER_NOTIFICATION_EMAIL_TO=toaddress@gmail.com \
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER=smtp.gmail.com \
+  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT=587 \  
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER=fromaddress@gmail.com \
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD=app_password \
   -e WATCHTOWER_NOTIFICATION_EMAIL_DELAY=2 \
@@ -199,4 +200,4 @@ docker run -d \
   -e WATCHTOWER_NOTIFICATION_URL="discord://token@channel slack://watchtower@token-a/token-b/token-c" \
   -e WATCHTOWER_NOTIFICATION_TEMPLATE="{{range .}}{{.Time.Format \"2006-01-02 15:04:05\"}} ({{.Level}}): {{.Message}}{{println}}{{end}}" \
   containrrr/watchtower
-```
+```