docs: ColumnOption::ForeignKey docs have wrong SQL syntax

https://docs.rs/sqlparser/0.55.0/sqlparser/ast/enum.ColumnOption.html#variant.ForeignKey

says

> A referential integrity constraint (`[FOREIGN KEY REFERENCES () { [ON DELETE ] [ON UPDATE ] | [ON UPDATE ] [ON DELETE ] } []`).

I believe the `FOREIGN KEY (name) REFERENCES` syntax is only for table constraints. That's why it has to say which source columns it talks about!

I believe the syntax for column constraints is just `col_name REFERENCES othertable (othercolumn)`.

As far as I can tell, the sqlparser-rs is already correct. For example, this won't parse:

```sql
-- wrong
CREATE TABLE myschema.mytable (
mycolumn BIGINT NOT NULL,
PRIMARY KEY(mycolumn),
bar BIGINT FOREIGN KEY REFERENCES foo (bar));
```

This does:

```sql
-- good
CREATE TABLE myschema.mytable (
mycolumn BIGINT NOT NULL,
PRIMARY KEY(mycolumn),
bar BIGINT REFERENCES foo (bar));
```

While we're at it, that big syntax literal should live in its own paragraph.

打开链接的 docs.rs 页面，并在仓库源代码中找到 ColumnOption::ForeignKey 的文档。根据 issue 中的示例验证列约束语法，然后修正语法描述，并将较大的语法字面量单独放在一个段落中。当生成的文档描述的是不带表约束 FOREIGN KEY 前缀的有效列语法时，即表示完成。

由索引模型根据 Issue 内容生成。