Slots syntax in slots-unification is wrong

[cathrinevaage]

In guide/migration/slots-unification, the syntax for passing slots to a component is incorrect.
RawSlots is not part of props, the second parameter in the used h overload, but part of third parameter children.
The syntax is correctly documented elsewhere: guide/render-function.

Suggested edit:

## 3.x Syntax

- In 3.x, render functions will have a `slots` option where they can be defined instead.
+ In 3.x, render functions can take an object of `slots` as the `children` parameter instead.

```js
// 3.x Syntax
- h(LayoutComponent, {
-   slots: {
-     header: () => h('div', this.header),
-     content: () => h('div', this.content)
-   }
+ h(LayoutComponent, {}, {
+   header: () => h('div', this.header),
+   content: () => h('div', this.content)
+ })
```

[Aferz]

PR: #365

We overlapped. Maintainers, feel free to close the PR if needed.

[NataliaTepluhina]

@cathrinevaage and @Aferz thank you both for the report and suggested change! 🙇🏻‍♀️