Update aggregates.md

[soerenwolfers]

SELECT 
mode() WITHIN GROUP (ORDER BY x),
mode(x),
mode(x ORDER BY x),
FROM 
(VALUES (2), (1)) _(x)

┌───────────────────────────────────┬─────────┬────────────────────┐
│ mode() WITHIN GROUP ( ORDER BY x) │ mode(x) │ mode(x ORDER BY x) │
│               int32               │  int32  │       int32        │
├───────────────────────────────────┼─────────┼────────────────────┤
│                                 1 │       2 │                  1 │
└───────────────────────────────────┴─────────┴────────────────────┘

shows that the WITHIN GROUP aggregate is converted to something more involved than what was previously claimed.

EDIT: Related ticket at duckdb/duckdb#11419, but I still think this PR makes the docs more correct than they currently are even without that ticket being resolved yet.

[szarnyasg]

Thanks! That's a clever use of <code> + | to make it format correctly in editors.