doc: remove paragraph break for upgrading to modules

doc: remove paragraph break for upgrading to modules

Previously, the release notes broke up the sentences that modules
is now ready for production use and where to file issues for
migration problems into separate paragraphs. This made it look like
the migration paragraph was about upgrading to 1.14, not to modules,
and made the reading a bit confusing. Now the entire idea is in one
paragraph.

Change-Id: I10bddfb8aba5f5909ac6842f25e3e97d505835e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/217720
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>