Trivial: Fix ISO URL, capitalization

- Changed Debian 8.5.0 ISO CD ROM URL by redirecting from current (which now has only 8.6.0) to a persistent archive link which should not change anytime soon.
- Added a link to official Debian checksum verification procedure (which is more verbose and also acts as a backup source of SHA256sum's)
- Fix capitalization (iso)

doc/gitian-building.md

@@ -95,14 +95,14 @@ After creating the VM, we need to configure it.
 
 - Click `Ok` twice to save.
 
-Get the [Debian 8.x net installer](http://cdimage.debian.org/debian-cd/8.5.0/amd64/iso-cd/debian-8.5.0-amd64-netinst.iso) (a more recent minor version should also work, see also [Debian Network installation](https://www.debian.org/CD/netinst/)).
-This DVD image can be validated using a SHA256 hashing tool, for example on
+Get the [Debian 8.x net installer](http://cdimage.debian.org/mirror/cdimage/archive/8.5.0/amd64/iso-cd/debian-8.5.0-amd64-netinst.iso) (a more recent minor version should also work, see also [Debian Network installation](https://www.debian.org/CD/netinst/)).
+This DVD image can be [validated](https://www.debian.org/CD/verify) using a SHA256 hashing tool, for example on
 Unixy OSes by entering the following in a terminal:
 
     echo "ad4e8c27c561ad8248d5ebc1d36eb172f884057bfeb2c22ead823f59fa8c3dff  debian-8.5.0-amd64-netinst.iso" | sha256sum -c
     # (must return OK)
 
-Then start the VM. On the first launch you will be asked for a CD or DVD image. Choose the downloaded iso.
+Then start the VM. On the first launch you will be asked for a CD or DVD image. Choose the downloaded ISO.
 
 ![](gitian-building/select_startup_disk.png)