fix small grammatical error in README.md (#2596)

Co-authored-by: Brage Sekse Aarset <brage.aarset@gmail.com>

Author: igorrocha

README.md

@@ -706,7 +706,7 @@ export class CustomTextLength implements ValidatorConstraintInterface {
 
 ## Custom validation decorators
 
-You can also create a custom decorators. Its the most elegant way of using a custom validations.
+You can also create a custom decorator. It's the most elegant way of using custom validations.
 Lets create a decorator called `@IsLongerThan`:
 
 1. Create a decorator itself: