Replace usages of depth buffer with depth attachment

[martty]

Depth buffer is an old term and not used by the Vulkan specification. The guide is clearer by not using it as well, improving the searchability of the term.

Also fixed a few typos I saw as I was editing.

[vblanco20-1]

Im not sure such a rename is a good thing, depth buffer is how everyone generally calls it, even if depth attachment is more accurate.

[martty]

Obviously not everybody, since Vulkan itself doesn't call it that - and this is a Vulkan tutorial. There is no point in teaching old language, it is just confusing.