Correct previous stable branch used in 8.14 to 8.15 update guide

[ci skip]

doc/update/8.14-to-8.15.md

@@ -113,7 +113,7 @@ sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS
 There are new configuration options available for [`gitlab.yml`](config/gitlab.yml.example). View them with the command below and apply them manually to your current `gitlab.yml`:
 
 ```sh
-git diff origin/8-13-stable:config/gitlab.yml.example origin/8-15-stable:config/gitlab.yml.example
+git diff origin/8-14-stable:config/gitlab.yml.example origin/8-15-stable:config/gitlab.yml.example
 ```
 
 #### Git configuration
@@ -131,10 +131,10 @@ Ensure you're still up-to-date with the latest NGINX configuration changes:
 
 ```sh
 # For HTTPS configurations
-git diff origin/8-13-stable:lib/support/nginx/gitlab-ssl origin/8-15-stable:lib/support/nginx/gitlab-ssl
+git diff origin/8-14-stable:lib/support/nginx/gitlab-ssl origin/8-15-stable:lib/support/nginx/gitlab-ssl
 
 # For HTTP configurations
-git diff origin/8-13-stable:lib/support/nginx/gitlab origin/8-15-stable:lib/support/nginx/gitlab
+git diff origin/8-14-stable:lib/support/nginx/gitlab origin/8-15-stable:lib/support/nginx/gitlab
 ```
 
 If you are using Apache instead of NGINX please see the updated [Apache templates].