Fix button hover color in cover example

[jtsimoes]

Hovering the button on the Cover example makes the text white, making it impossible to read on the white background.

This is due to the use of .btn-secondary class (which applies color: var(--bs-btn-hover-color) aka #fff on hover) together with the use of .bg-white class.

This PR fix this issue. I decided to simply switch from .btn-secondary to the .btn-light class, since it is the closest thing to a white button that we have at the moment.

Thus, this class changes the button text on hover to black, providing also a nice transition effect (from #333 to #000) that did not exist before.

v5.1	v5.2	This PR