DOC: Update ITK version mentioned in download.md from 4.13.1 to 5.3.0

Download is now the second page of the new docs site (docs.itk.org), so an update to it is in order.
The direct link is:
https://docs.itk.org/en/latest/download.html

Author: dzenanz

Documentation/docs/download.md

@@ -24,7 +24,7 @@ ITK packages exist for many Linux distributions. For Debian and Ubuntu, they
 can be obtained running
 
 ```sh
-   $ sudo apt-get install libinsighttoolkit4-dev
+   $ sudo apt-get install libinsighttoolkit5-dev
 ```
 
 This will give you the libraries and development headers.
@@ -77,9 +77,9 @@ where `ITKLatestRelease` is the name of the local repository created.
 Additionally, specific releases can be cloned using the tags, for example:
 
 ```sh
-   $ git clone -b v4.13.1 https://github.com/InsightSoftwareConsortium/ITK ITK-4.13.1
+   $ git clone -b v5.3.0 https://github.com/InsightSoftwareConsortium/ITK ITK-5.3.0
 ```
-where `v4.13.1` corresponds to ITK 4.13.1, and the `ITK-4.13.1` is the name
+where `v5.3.0` corresponds to ITK 5.3.0, and the `ITK-5.3.0` is the name
 of the local repository created.
 
 ### Update
@@ -114,7 +114,7 @@ instructions to update it.
 Alternatively, one may checkout a specific release tag using
 
 ```sh
-   $ git checkout v4.13.1
+   $ git checkout v5.3.0
 ```
 
 Release tags never move. Repeat the command with a different tag to get a