fix bug

tasks/MD5.wdl

 task MD5 {
 
 	File file
-	String basename = basename(file)
 	String docker
 	String cluster_config
 	String disk_size
 
 	command <<<
-		md5sum ${file} > ${basename}.md5
+		md5sum ${file} > $(basename ${file}).md5
 	>>>
 
 	runtime {
 	}
 
 	output {
-		String MD5 = "${basename}.md5"
+		String MD5 = "$(basename ${file}).md5"
 	}
 }