Fix typo in docs

doc/user/project/pages/getting_started_part_three.md

@@ -149,7 +149,7 @@ verify your domain's ownership with a TXT record:
 Once you've set the DNS record, you'll need navigate to your project's
 **Setting > Pages** and click **+ New domain** to add your custom domain to
 GitLab Pages. You can choose whether to add an [SSL/TLS certificate](#ssltls-certificates)
-to make your website accessible under HTTPS or leave it blank. If don't add a certificate,
+to make your website accessible under HTTPS or leave it blank. If you don't add a certificate,
 your site will be accessible only via HTTP:
 
 ![Add new domain](img/add_certificate_to_pages.png)