qapi: Fix comment for create-type to match code.

Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qapi-schema.json

@@ -227,7 +227,7 @@
 ##
 # @ImageInfoSpecificVmdk:
 #
-# @create_type: The create type of VMDK image
+# @create-type: The create type of VMDK image
 #
 # @cid: Content id of image
 #