1.12 release timeline fixups
1.12 release timeline fixups

An astute observer has noted some editing gaps on the timeline, which
are fixed here.  The initial text and table were correct, but the later
definitions sections had a couple off-by-one month issues.

Fixes #227

Signed-off-by: Tim Pepper <4807564e5a18784afafcf20dbdb5591e844b9075@vmware.com>

kubernetes/sig-release