fix: small grammatical error on README.md

[igorrocha]

Description

In the README section about custom decorators, it says: "You can also create a custom decorators. Its the most elegant way of using a custom validations.". The correct grammar would be "You can also create a custom decorator. It's the most elegant way of using custom validations."

Minimal code-snippet showcasing the problem

// Not applicable. Documentation changes only.

Expected behavior

Not applicable. Documentation changes only.

Actual behavior

Not applicable. Documentation changes only.