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.

Author: loganmay

docs/docs/themes/conventions.md

@@ -7,7 +7,7 @@ As methodologies for building Gatsby Themes begin to formalize and standardize,
 ## Naming
 
 It's required to prefix themes with `gatsby-theme-`. So if you'd like to name your theme "awesome" you
-can name it `gatsby-theme-awesome` and place that as the `name` key in your `package.json`. Prefixing themes with `gatsby-theme` enables Gatsby in identifying theme packages for compilation.
+can name it `gatsby-theme-awesome` and place that as the `name` key in your `package.json`. Prefixing themes with `gatsby-theme-` enables Gatsby in identifying theme packages for compilation.
 
 ## Initializing required directories