updated README screenshot reference

Author: mariodavid

README.md

@@ -65,7 +65,7 @@ In order to send a message to a particular user, there is a button "Send message
 
 This screen allows to manually send a message. A message contains a subject and a body, just like a regular Email.
 
-![Screenshot of sending a new message](https://github.com/mariodavid/cuba-component-user-inbox/blob/master/img/send-messages.png)
+![Screenshot of sending a new message](https://github.com/mariodavid/cuba-component-user-inbox/blob/master/img/send-message.png)
 
 
 This feature can sometimes be helpful but oftentimes sending a regular email is not worse.
@@ -127,4 +127,4 @@ One of these two classes can be used as the mainwinow through the following defi
 
 You can also extend this screens, so that you can add your own (screen-) logic to the mainwindow.
 
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+