docs: use plural names instead of singular

[KhoDis]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

This PR standardizes class naming to the plural form, as recommended in #1176.

What is the new behavior?

Changes:

• Updated UserService to UsersService in prisma.md and other similar cases.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Fixes #1176
Fixes #3220

[kamilmysliwiec]

lgtm