Be consistent about java_heap_dump parameters.

Change-Id: Ib8595f4d81eea11521aac6c942a09a9f1a2f280e
diff --git a/tools/java_heap_dump b/tools/java_heap_dump
index 178ded6..ced597e 100755
--- a/tools/java_heap_dump
+++ b/tools/java_heap_dump
@@ -105,10 +105,11 @@
       action="store_true",
       help="Print config instead of running. For debugging.")
   parser.add_argument(
-      "--stop_when_done",
+      "--stop-when-done",
       action="store_true",
       help="On recent builds of S, use a new method to stop the profile when "
            "the dump is done. Previously, we would hardcode a duration.")
+
   args = parser.parse_args()
 
   fail = False