fix(a11y): fix argments for initNavEl() method

[yuheiy]

When I using the a11y module, the aria-label and aria-controls values applied to the navigation buttons are incorrect.

Reproduction code:

import Swiper, { A11y, Navigation } from "swiper";

new Swiper(".swiper", {
  modules: [A11y, Navigation],
  a11y: {},
  navigation: {
    nextEl: ".swiper-button-next",
    prevEl: ".swiper-button-prev",
  },
});

<div class="swiper">
  <div class="swiper-wrapper">
    <div class="swiper-slide">Slide 1</div>
    <div class="swiper-slide">Slide 2</div>
    <div class="swiper-slide">Slide 3</div>
  </div>
  <button class="swiper-button-prev" type="button">Previous</button>
  <button class="swiper-button-next" type="button">Next</button>
</div>

Result:

<button class="swiper-button-prev swiper-button-disabled" disabled="" tabindex="-1" aria-label="undefined" aria-controls="Previous slide" aria-disabled="true">Previous</button>
<button class="swiper-button-next" tabindex="0" aria-label="undefined" aria-controls="Next slide" aria-disabled="false">Next</button>

The reason for aria-label="undefined", aria-controls="Previous slide" and aria-controls="Next slide" is that the arguments given to initNavEl() are wrong.

[nolimits4web]

Merged, thanks!