clarify RFC3339 parsing, recommend ciso8601, remove fractional second…

[philvarner]

…s comma separator, which is not valid in 3339

Related Issue(s): #197

Proposed Changes:

1. fix invalid description of RFC 3339 that indicates a , is valid for the fractional seconds separator
2. recommend ciso8601

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.