Missing `@map` for fields example

[keerlu]

Add @map for fields to schema data model page to match @map for models

From @janpio:

I think we are missing an @map for fields example on https://www.prisma.io/docs/concepts/components/prisma-schema/data-model (We do have https://www.prisma.io/docs/concepts/components/prisma-schema/data-model#mapping-model-names-to-tables-or-collections for tables/models) (We also have something for fields here: https://www.prisma.io/docs/concepts/components/prisma-schema/names-in-underlying-database#map-field--column-names)

Slack link: https://prisma-company.slack.com/archives/C5Z9TH6N9/p1655967600496789