Update compute_build_timestamp.py comment

compute_build_timestamp.py was updated to not do 5 am quantizing on
official builds but the behavioral comment at the top was not updated.
This change just updates the comment to reflect reality.

Also fix a spelling error found by tricium.

Bug: 993509
Change-Id: I05e3c03d5846f32a47b7742e01739afd5d4f0564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154532Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760120}

build/compute_build_timestamp.py

@@ -8,9 +8,8 @@ build_type impacts the timestamp generated, both relative to the date of the
 last recent commit:
 - default: the build date is set to the most recent first Sunday of a month at
   5:00am. The reason is that it is a time where invalidating the build cache
-  shouldn't have major reprecussions (due to lower load).
-- official: the build date is set to the current date at 5:00am, or the day
-  before if the current time is before 5:00am.
+  shouldn't have major repercussions (due to lower load).
+- official: the build date is set to the time of the most recent commit.
 Either way, it is guaranteed to be in the past and always in UTC.
 """