chore: fix PrefixSetMut doc comment

[Rjected]

Previously the comment said that PrefixSetMut used a BTreeSet, which is no longer true, so the comment is updated with how it works currently.

[codecov]

Codecov Report

Merging #4015 (8ada862) into main (27c65d2) will decrease coverage by 0.02%.
The diff coverage is n/a.

Files Changed	Coverage Δ
crates/trie/src/prefix_set/mod.rs	83.11% <ø> (ø)

... and 10 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.24% <ø> (+<0.01%)	⬆️
unit-tests	64.16% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	25.63% <ø> (ø)
blockchain tree	83.04% <ø> (ø)
pipeline	90.03% <ø> (ø)
storage (db)	74.30% <ø> (ø)
trie	94.70% <ø> (ø)
txpool	46.00% <ø> (+0.60%)	⬆️
networking	77.47% <ø> (-0.18%)	⬇️
rpc	58.35% <ø> (-0.03%)	⬇️
consensus	63.51% <ø> (ø)
revm	33.10% <ø> (ø)
payload builder	6.58% <ø> (ø)
primitives	88.06% <ø> (-0.03%)	⬇️

[rkrasiuk]

lgtm, thanks