Improve createPages of blog starter (#10781)
Improve createPages of blog starter (#10781)

## Description
Remove extra promise in `starters/blog/gatsby-node.js`.

## Related Issues
The same fixes is made for these pull requests.

https://github.com/gatsbyjs/gatsby/pull/10357
https://github.com/gatsbyjs/gatsby/pull/10777

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