added missing sources plugin

Author: wumpz

Diff for: pom.xml

@@ -111,7 +111,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-source-plugin</artifactId>
-                <version>4.0.0-beta-1</version>
+                <version>3.3.1</version>
                 <executions>
                     <execution>
                         <id>attach-sources</id>