Update and rename EntityHashSet 0.16 migration section

[guyguy2001]

Solves bevyengine/bevy#18841

I also renamed the md file, since the name seems like a mistake to me.

I didn't test the md - should I compile the docs locally? Or is a review enough?

The 0.16 doesn't get any rust analyzer errors and go to definition works for it, and the 0.15 version compiles. (I didn't have a compiling 0.16 projects at hand)

[guyguy2001]

Also, I never used these structs before (I'm toying with upgrading bevy_tnua to 0.16, but I never touched it before) - so I may have missed important context in this migration tip, which might make my change wrong. Specifically, I haven't tested that my modified 0.15 line actually works in 0.15, since I don't have a project of that version on hand.

[JaySpruce]

This correction is correct for bevyengine/bevy#17447, but it's out of date for 0.16 because the re-exports were re-introduced by bevyengine/bevy#18413. There's no longer any change from a user's perspective going from 0.15.3 to 0.16, so this migration guide ought to just be removed

[guyguy2001]

Updated, and tested locally that the paragraph was indeed missing - was wonderfully simple.

And the 0.15 example indeed compiles for me on 0.16.0 rc 5.