[Website] grid item reference type is probably not correct

[bradh]

Summary

mp4ra.github.io/data/item-references.csv

Line 12 in 31087a5

grid,Image item grid reference,item ref.,HEIF

shows grid as an item reference. I don't think ISO/IEC 23008-12:2022 says that. It does describe an item_type of grid (which I assume is as registered at

mp4ra.github.io/data/item-types.csv

Line 7 in 31087a5

grid,Image item grid derivation,HEIF

).

Instead, grid items use dimg item references, as shown in Section 6.6.2.3.1.

Steps to reproduce

1. See above.