docs(sheets): fix invalid +dim-insert example in chart reference

The chart reference's placement example used non-existent flags
--dimension/--start/--end for +dim-insert. The real signature is
--position (required) + --count (required); copying the example
fails Validate with "--position is required". Replace it with
+dim-insert --position V --count 6 (insert 6 columns before V,
i.e. after U), aligning with the sheet-structure reference.

skills/lark-sheets/references/lark-sheets-chart.md

@@ -95,7 +95,7 @@
 **示例**：21 列 sheet 放 600×400 图 → `needCols=6, needRows=15`
 - ❌ `{row: 0, col: "W"}` — col=22 越界
 - ✅ `{row: 42, col: "A"}` — 放数据下方
-- ✅ 先 `+dim-insert --dimension column --start 21 --end 27`（在 U 列后插 6 列；U=index 20，after 即从 21 起），再放图到 `{row: 0, col: "V"}`
+- ✅ 先 `+dim-insert --position V --count 6`（在 V 列前插 6 列，即 U 列之后），再放图到 `{row: 0, col: "V"}`
 
 ## Shortcuts