Update section "Proponents" 5.2

[SteveLLamb]

Currently reads:

The [SMPTE Standards Operation Manual v3.1](https://github.com/SMPTE/ag-06-1/issues/new#bib-smpte-standards-operation-manual-v31) §6.3 requires one or more Proponents, who are SMPTE Standards Members agreeing to be actively involved in the Project

Should be updated to reflect this as per 6.3 of the OM:

Except in the case of an Registered Disclosure Document, each Project shall be supported by at least two Technology Committee Members representing different organizations; and

[SteveLLamb]

To be resolved when OM is updated.