Create substitution rule for GitLab Shell

Add an error-level substitution rule to prevent users from writing
the product name as "GitLab-shell" - capitalization is already
handled in .markdownlint.json

doc/.vale/gitlab/Substitutions.yml

@@ -13,6 +13,7 @@ ignorecase: true
 swap:
   frontmatter: front matter
   GitLabber: GitLab team member
+  GitLab-shell: GitLab Shell
   gitlab omnibus: Omnibus GitLab
   param: parameter
   params: parameters

doc/development/internal_api.md

@@ -52,7 +52,7 @@ POST /internal/allowed
 | `username` | string | no      | Username from the certificate used to connect to GitLab Shell |
 | `project`  | string | no (if `gl_repository` is passed) | Path to the project |
 | `gl_repository`  | string | no (if `project` is passed) | Repository identifier (e.g. `project-7`) |
-| `protocol` | string | yes     | SSH when called from GitLab-shell, HTTP or SSH when called from Gitaly |
+| `protocol` | string | yes     | SSH when called from GitLab Shell, HTTP or SSH when called from Gitaly |
 | `action`   | string | yes     | Git command being run (`git-upload-pack`, `git-receive-pack`, `git-upload-archive`) |
 | `changes`  | string | yes     | `<oldrev> <newrev> <refname>` when called from Gitaly, the magic string `_any` when called from GitLab Shell |
 | `check_ip` | string | no     | IP address from which call to GitLab Shell was made |