Update the screenshot

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

appinfo/info.xml

@@ -28,7 +28,7 @@
 	<website>https://github.com/nextcloud/mail#readme</website>
 	<bugs>https://github.com/nextcloud/mail/issues</bugs>
 	<repository type="git">https://github.com/nextcloud/mail.git</repository>
-	<screenshot>https://raw.githubusercontent.com/nextcloud/mail/74e94da16618b32ee0834e57bbfc83ff7334f709/screenshots/mail.png</screenshot>
+	<screenshot>https://user-images.githubusercontent.com/1374172/79554966-278e1600-809f-11ea-82ea-7a0d72a2704f.png</screenshot>
 	<dependencies>
 		<php min-version="7.2" max-version="7.4" />
 		<nextcloud min-version="17" max-version="19" />