selftests/vm/run_vmtests: Fix wrong comment

A comment in `run_vmtests` is wrong because it is saying `128MB + 128MB
== 258MB`.  This commit fixes the comment.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>