[sandbox] Update test/mkgrokdump/README

When regenerating v8heapconst.py, the v8 sandbox now has to be enabled
explicitly (using the v8_enabe_sandbox=true gn arg) as it is enabled by
default in Chromium builds, but not standalone v8 builds.

Bug: v8:13281
Change-Id: I1a0861b1d63f340465d7433e042b27eace706ca7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3895622
Commit-Queue: Adam Klein <adamk@chromium.org>
Auto-Submit: Samuel Groß <saelo@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83201}

test/mkgrokdump/README

 If you change the heap layout, expect this test to fail (in CQ). You will then
-need to build x64 release and run `<outdir>/mkgrokdump > tools/v8heapconst.py`
-to rebaseline.
+need to build x64 release with v8_enable_sandbox=true and run
+`<outdir>/mkgrokdump > tools/v8heapconst.py` to rebaseline.
+The V8 sandbox has to be manually enabled as it is enabled by default only for
+Chromium builds, but not standalone v8 builds.