Fix SAR extension example

[antoniomo]

Proposed Changes:

Just a fix for this SAR extension example, as sar:bands is no longer part of the spec.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.