Update buttons.md

[FlamongoBird]

Two colons were missing in the flutter docs, Whoever wrote those docs needs to be fired, this is not acceptable.

Description of what this PR is changing or adding, and why:

This is the most important PR ever...
Literally just two colons that were missing.

Issues fixed by this PR (if any):

Fixed two missing colons.

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[sfshaza2]

/gcbrun

[sfshaza2]

lgtm

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 5a7e1ec):

https://flutter-docs-prod--pr11691-patch-1-qxsiuqhe.web.app