Fix typo update-plans.md

[v4nft]

"than" was incorrectly used instead of "then". Also made a couple of other grammatical improvements.

Description

Checklist:

• I have reviewed the style guide.
• If blogging, I have reviewed the blogging guide.
• I have manually confirmed that all new links work.
• I added aliases (i.e., redirects) for all filename changes.
• If making css changes, I rebuilt the bundle.