Change crostini installer cancel screen message to match UX guidance

Bug: 930583
Change-Id: I35563d31bcadd88a9f134595eaf6fc749f38e4e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575306
Commit-Queue: Fergus Dall <sidereal@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654303}
2 files changed