Mark 9+ for Debian version

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docs/getting-started/installation.md

@@ -6,7 +6,7 @@ Dokku is an extensible, open source Platform as a Service that runs on a single
 
 To start using Dokku, you'll need a system that meets the following minimum requirements:
 
-- A fresh installation of [Ubuntu 16.04/18.04 x64](https://www.ubuntu.com/download), [Debian 9 x64](https://www.debian.org/distrib/) or [CentOS 7 x64](https://www.centos.org/download/) *(experimental)* with the FQDN set <sup>[1]</sup>
+- A fresh installation of [Ubuntu 16.04/18.04 x64](https://www.ubuntu.com/download), [Debian 9+ x64](https://www.debian.org/distrib/) or [CentOS 7 x64](https://www.centos.org/download/) *(experimental)* with the FQDN set <sup>[1]</sup>
 - At least 1 GB of system memory <sup>[2]</sup>
 
 You can *optionally* have a domain name pointed at the host's IP, though this is not necessary.
@@ -27,7 +27,7 @@ sudo DOKKU_TAG=v0.19.13 bash bootstrap.sh
 
 The installation process takes about 5-10 minutes, depending upon internet connection speed.
 
-If you're using Debian 9 or Ubuntu 18.04, make sure your package manager is configured to install a sufficiently recent version of nginx<sup>[3]</sup>, otherwise, the installation may fail due to `unmet dependencies` relating nginx.
+If you're using Debian 9+ or Ubuntu 18.04, make sure your package manager is configured to install a sufficiently recent version of nginx<sup>[3]</sup>, otherwise, the installation may fail due to `unmet dependencies` relating nginx.
 
 #### 2. Setup SSH key and Virtualhost Settings