fix: make description in Item Search openapi GET/POST impl consistent…

[philvarner]

Related Issue(s): #167

Proposed Changes:

1. make optional/required language for GET/POST in openapi yaml consistent with README

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.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[cholmes]

A good improvement. Funny, I came across this language a few days ago and was definitely confused by it (of course there's a good chance I wrote it...). This is much clearer.