docs(gatsby-remark-custom-blocks): fix class names configurations to match example outputs (#10631)
docs(gatsby-remark-custom-blocks): fix class names configurations to match example outputs (#10631)

## Description

In [`gatsby-remark-custom-blocks`](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-remark-custom-blocks) docs outputed class names are wrong, because if we set this:
```js
options: {
  blocks: {
    danger: {
      classes: "custom-block-danger"
    // ...
}
```
we will get classes in html: `custom-block custom-block-danger` and not `custom-block-danger` only as in docs.

This PR fix the confusion

gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby