All the terms in "Terms defined by reference" in the Index are spelled in all lowercase

All the terms in ["Terms defined by reference"](https://aomediacodec.github.io/av1-avif/#index-defined-elsewhere) in the Index are spelled in all lowercase, for example, "av1 bitstream", "frameheight", and "metadata obu". These should be spelled as "AV1 bitstream", "FrameHeight", and "metadata OBU", respectively.

