use less memory on bcftools stats.

bcftools.wdl

@@ -86,7 +86,7 @@ task Stats {
 
         Int threads = 0
         Int timeMinutes = 1 + 2* ceil(size(select_all([inputVcf, compareVcf]), "G"))  # TODO: Estimate, 2 minutes per GB, refine later.
-        String memory = "2G"  # TODO: Safe estimate, refine later.  
+        String memory = "256M" 
         String dockerImage = "quay.io/biocontainers/bcftools:1.9--ha228f0b_3"
     }