PartialEq example is not symmetric

[RalfJung]

With #51760 landed, the PartialEq documentation now at the same time states

Formally, the equality must be (for all a, b and c):

• symmetric: a == b implies b == a; and
• [...]

And gives an example that is not symmetric: b1 == BookFormat::Paperback typechecks, but BookFormat::Paperback == b1 does not.

Cc @gnzlbg @GuillaumeGomez @steveklabnik

[gnzlbg]

I can fix this in #53386 .

[steveklabnik]

Triage: that PR was never merged, so this is still open.

[frewsxcv]

Opened a PR for this #57357