fix: example on "always" option in doc

[Pernet]

Description

Documentation of "Validation groups" in README should be more explicit for "always" behavior.

import { validate, Min } from 'class-validator';

export class Foo {
  @Min(2, { groups: ['ko' })
  p_ko!: number;
  @Min(2, { groups: ['ok'] })
  p_ok!: number;
  @Min(2)
  p_no_group!: number;
  @Min(2, { always: true, groups: ['ignored'] })
  p_always!: number;
  p_none?: number;
}

let foo = new Foo();
foo.p_ko = 1;
foo.p_ok = 2;

foo.p_no_group = 2;
foo.p_always = 2;
validate(foo, { groups: undefined }); // validation: false on 'p_ko' property
validate(foo, { groups: [] });        // validation: false on 'p_ko' property
validate(foo, { groups: ['ko'] });    // validation: false on 'p_ko' property
validate(foo, { groups: ['ok'] });    // validation: true

foo.p_no_group = 1;
foo.p_always = 1;
validate(foo, { groups: undefined }); // validation: false on 'p_ko' and 'p_no_group' and 'p_always' properties
validate(foo, { groups: [] });        // validation: false on 'p_ko' and 'p_no_group' and 'p_always' properties
validate(foo, { groups: ['ko'] });    // validation: false on 'p_ko' and                  'p_always' properties
validate(foo, { groups: ['ok'] });    // validation: false on                             'p_always' property