Fixed standard symbol use in unitsystem definitions.

[oke464]

Changed symbols used in unitsystem definitions to be consistent with the standard symbol defined in the referenced unit data schema.

[ml-evs]

I think I'm a bit out of the loop on this one, I assume its correct but perhaps @rartino can confirm? (As I guess the schemas at schemas.optimade.org will also need to be regenerated?)

[vaitkus]

Looks good to me as well.

@oke464, could you please also fix one minor unrelated issue in the schemas/src/defs/v1.2/unitsystems/si/si_1985.yaml file? The description field incorrectly states the release date as "in 1995" instead of "in 1985".

[rartino]

Thanks @oke464! (This wasn't caught with the schemas and sanity-checks as it is an intentional feature that a unitsystem can be defined to include a unit but with a non-standard symbol; it just wasn't the intent to use that feature here.)

I took the liberty to also add the fix @vaitkus asked for and, while we are anyway doing an update, a couple of minor typographical adjustments I noticed since the big merge.

@ml-evs This indeed affected a couple of definitions/pages, in particular for unitsystems, under schemas.optimade.org. I've now updated them.