Plasmid documentation mentions removed MoreCodecs.TEXT codec

[haykam821]

The MoreCodecs.TEXT field was removed in NucleoidMC/more-codecs@04e9bd9, but is still mentioned here:

nucleoid-docs/docs/plasmid/getting-started.md

Line 86 in 48989fb

- Codec tip: most serializable Minecraft types will hold a static `CODEC` field for use (e.g. `BlockPos.CODEC` or `Identifier.CODEC`). If not, we bundle a `MoreCodecs` type which provides some common ones that are not included in the vanilla codebase (e.g. `MoreCodecs.TEXT`).

This issue corresponds to NucleoidMC/plasmid#286, as the README there is kept in sync with the documentation here.