added upper-case-solution

This solves my issue github#8269

data/reusables/pages/create-repo-name.md

@@ -1,2 +1,3 @@
-1. Type a name for your repository and an optional description. If you're creating a user or organization site, your repository must be named `<user>.github.io` or `<organization>.github.io`. For more information, see "[About {% data variables.product.prodname_pages %}](/articles/about-github-pages#types-of-github-pages-sites)."
+1. Type a name for your repository and an optional description. If you're creating a user or organization site, your repository must be named `<user>.github.io` or `<organization>.github.io`. If `<user>` or `<organization>` contain uppercase letters, you need to lowercase them.
+For more information, see "[About {% data variables.product.prodname_pages %}](/articles/about-github-pages#types-of-github-pages-sites)."
    ![Create repository field](/assets/images/help/pages/create-repository-name-pages.png)