docs(IsDecimal): use correct docstring for IsDecimal

[braaar]

Description

Use the correct docstring for the decorator IsDecimal. The existing one seems to have been copied from IsAlphanumeric: IsAlphanumeric source code

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
• [x] tests are added for the changes I made (if any source code was modified)
• [x] documentation added or updated
• [ ] I have run the project locally and verified that there are no errors

Fixes

fixes #1423

[braaar]

I think the merge commit made the Semantic Pull Request check fail?

[braaar]

As long as we are using squash and merge I don't see any issue with the PR.

[NoNameProvided]

Landed in https://github.com/typestack/class-validator/commit/42b9ca263cf855d2004c0dce23e38d23546148ac

[github-actions[bot]]

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