graphics/opencolorio: update to 2.1.3

• pkg-plist adjustment for new sphinx version
• remove DOCS from OPTIONS_DEFAULT as it is implied

Changelog: https://github.com/AcademySoftwareFoundation/OpenColorIO/releases/tag/v2.1.3

PR: 269186
Reported by: Robert Cina <transitive@gmail.com>
Approved by: flo (mentor)
Differential Revision: https://reviews.freebsd.org/D38633