change GENE_ID to gene_id in ballgown task

tasks/ballgown.wdl

@@ -11,7 +11,7 @@ task ballgown {
     cp -r ${sep=" " ballgown} /cromwell_root/tmp/${base}
     ballgown /cromwell_root/tmp/${base} ${base}.txt
     sed -i 's/"//g' ${base}.txt
-    sed -i '1s/FPKM./GENE_ID\t/g' ${base}.txt
+    sed -i '1s/FPKM./gene_id\t/g' ${base}.txt
   >>>
   
   runtime {