Add - to end of second `gatsby-theme` prefix str (#22523)
Add - to end of second `gatsby-theme` prefix str (#22523)

The maximal prefix is `gatsby-theme-` so make sure both instances of that string include the final `-` for consistency.
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby