Replace img-fluid by w-100 in Carousel docs to conform with HTML example

Created by: webchopin

Here is the documentation part concerned: https://getbootstrap.com/docs/4.0/components/carousel/#slides-only

Before this PR, there was a mismatch, in Carousel docs - "Slides only" section, between the introduction stating that img-fluid is present and the code example with w-100 instead. So, we use w-100 to conform with the code example and to be consistent with this commit: https://github.com/twbs/bootstrap/pull/22392/commits/0857713a38444bf76bb1a51183aed8a0c26fe06f