docs: Update deprecated function names in README and docs

[sixtusagbo]

Description

Rename the deprecated functions in the README and the files in the docs directory to match the current functions which better reflect what they are doing.

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

Fixes

fixes #[issue number], fixes #[issue number]

[vaxul]

Today I stumbled over this fact, that the docs says plainToClass, even it's deprecated and replaced by plainToInstance.