Docs: correction in README.md codeblocks, fix error "validationOptions can not be undefined" when creating custom class validators

[ashuvssut]

when creating custom class validators, validationOptions can not be undefined.
It gives off this error: Type 'undefined' is not assignable to type 'ValidationOptions'. (REF issue #1642)

Correction in documentation: the validationOption argument from custom validation decorator functions is now not allowed to be optional.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• 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)
• documentation added or updated

fixes #1642

[NoNameProvided]

The change is invalid, the parameter is optional.

[ashuvssut]

This PR was created for Issue #1642. I have given my issue details there in that Issue

But, this issue is too old now and I am no longer working on that project. Therefore I think I will be closing issue #1642.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.