docs: Correct place for passing `extension` option to `createMDX()` (#55967)
### What?
Fixing a little mistake in the documentation, where the placement of the `extension` option for `createMDX()` was described as being inside the `options` property:
```ts
const withMDX = createMDX({
options: {
extension: /\.mdx?$/,
```
But it's actually a property of its own:
```ts
const withMDX = createMDX({
extension: /\.mdx?$/,
```
I confirmed that the latter is correct because:
- It solves an issue I was having: Unable to import `.md` files
- On the same docs page, there's another place where it mentions this `extension` option and the code example is correct there
- The option is described in a similar issue, such as https://github.com/vercel/next.js/issues/45431#issuecomment-1410363864
next.js
9d86c122 - docs: Correct place for passing `extension` option to `createMDX()` (#55967)
