Update "help" metadata in Sdr

[sebastienblor]

We're currently using the arnold "desc" metadata to provide USD registry a "Help" value.
However, this is changing in the arnold side, as the metadata is now called "help", so we need to update the ndr parser code

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-14681

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-14725