Documentation: Update the default value of crashkernel low

In commit 94fb9334

 ("x86/crash: Allocate enough low memory when
crashkernel=high") the default value of crashkernel low memory
is changed to 256M. In this patch update it accordingly.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>