Docs: Example code of adding innerBlocks to Block Preview is incorrect.

[naogify]

Describe the bug

The example section in block registration document, it use object expression, like innerBlocks: { ... } to adding innerBlocks for previewing.
I got Cannot read property 'attributes' of undefined error, by following the way above.

But, I found the the core/cover block use array expression, like innerBlocks: [ ... ] . It works well.
https://github.com/WordPress/gutenberg/blob/master/packages/block-library/src/cover/index.js#L32

We should fix the example code in the document , from innerBlocks: { ... }. to innerBlocks: [ ... ].

To reproduce
Adding innerBlocks for block example in block registration by innerBlocks: { ... } .

Expected behavior
Can see Block Preview.

Screenshots
none

Editor version (please complete the following information):

• WordPress version: 5.4.2
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default?: "default"

Desktop (please complete the following information):

• OS: [macOS Mojave]
• Browser [chrome]
• Version [10.14.6]

Smartphone (please complete the following information):
none

Additional context
none