update description for bbox property in item-spec.md

[nkleinbaer]

Related Issue(s): #
None

Proposed Changes:

1. Update the description for the bbox property in item-spec.md to indicate that bbox is forbidden when geometry is null

If this was a new requirement it would be a breaking change, but IMO this is simply an editorial change to make the markdown more accurately describe a requirement that is already present in the schema:

stac-spec/item-spec/json-schema/item.json

Lines 67 to 80 in fd8fb4e

	{
	"type": "object",
	"required": [
	"geometry"
	],
	"properties": {
	"geometry": {
	"type": "null"
	},
	"bbox": {
	"not": {}
	}
	}
	}

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.

[m-mohr]

Thank you.