Fix post-installation code blocks

Markdown code blocks apparently need 4 spaces; 3 is not enough.

README.md

@@ -66,15 +66,15 @@ make gitlab_repo=git@gitlab.com:example/gitlab-ce.git gitlab_shell_repo=git@gitl
 You can start Redis & Postgresql by running the command below, and keeping it
 running.
-   bundle exec foreman start
+    bundle exec foreman start
 To seed the main GitLab database:
-   cd gitlab && bundle exec rake db:create dev:setup
+    cd gitlab && bundle exec rake db:create dev:setup
 Finally, you can start the main GitLab rails application with:
-   bundle exec foreman start
+    bundle exec foreman start
 END Post-installation