Try lowercase rc

Signed-off-by: Joas Schilling <coding@schilljs.com>

appinfo/info.xml

@@ -16,7 +16,7 @@ And in the works for the [coming versions](https://github.com/nextcloud/spreed/m
 
 	]]></description>
 
-	<version>6.0.0-RC.1</version>
+	<version>6.0.0-rc.1</version>
 	<licence>agpl</licence>
 
 	<author>Daniel Calviño Sánchez</author>