fix: README.md update for instanceToPlain, plainToInstance and instan…

[alanmareines]

Description

Updates README.md file according to CHANGELOG.md:

• classToPlain -> instanceToPlain
• plainToClass -> plainToInstance
• classToClass -> instanceToInstance

Checklist

• [x] the pull request title describes what this PR does (not a vague title like Update index.md)
• [x] the pull request targets the default branch of the repository (develop)
• [x] the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• [ ] tests are added for the changes I made (if any source code was modified)
• [x] documentation added or updated
• [x] I have run the project locally and verified that there are no errors

[diffy0712]

@braaar I think this pr should be merged as it just updates the documentation to reflect the current state of the code.

[diffy0712]

Fixed in https://github.com/typestack/class-transformer/pull/1541