Update secured asset transfer tutorial (#3503)

Updates to align with latest sample:
- Set state-based endorsement to asset owner only
- Document that additional endorsers could be set to make the sample more secure, e.g. using a trusted third party
- Set environment variable for ASSET_ID and reference in commands to make it easy to copy/paste sample commands

Signed-off-by: David Enyeart <enyeart@us.ibm.com>