Rename metaschema's `$defs/simpleTypes` to singular

[notEthan]

This is somewhat trivial but I think this is more correct. The string is a simple type, not multiple types.

[gregsdennis]

I think it's correct as is. It's common to name an enumeration type as the plural, effectively stating, "There are the available X." In this case, this definition is listing the available types.

[notEthan]

All the other $defs names refer to the contents of the instance, not what's in the schema. I have only ever seen conventions of naming $defs names per the instance described. But, I thought this would be trivial/noncontroversial, not particularly wanting to debate it, so if that is not accurate, I am not bothered if you close this (or I can).

[gregsdennis]

I'm happy to get other opinions, but plural makes most sense to me.