README: drop header: option doc

It was undocumented in practice and only added confusion. The option
still exists in code for backward-compat but is no longer advertised.

Author: Fivell

README.md

@@ -82,7 +82,6 @@ May 18 → 21 booking shows on 18 (`→`), 19 (`·`), 20 (`←`).
 |---|---|
 | `group_by:` (Symbol) | Bucket by a date/datetime column. **1 SQL** per month (prefetched). |
 | `group_by_scope:` (Symbol) | Call `Model.scope(date)` per cell — for ranges, joins, time-zone math. **N SQL** per month (one per visible day). |
-| `header:` (String / Proc) | Rendered next to the month title. Proc receives `(collection, current_month)`. |
 
 URL params `?year=2026&month=5` drive the visible month. Today / Previous /
 Next links are rendered automatically and preserve current filter params.