Add Debian 8 to list of supported versions (#4907)

It was already present in the list further down; this merely adds it to the header.

Author: perlun

reference/docs-conceptual/install/Installing-PowerShell-Core-on-Linux.md

@@ -7,7 +7,7 @@ ms.date: 07/19/2019
 # Installing PowerShell Core on Linux
 
 Supports [Ubuntu 16.04][u16], [Ubuntu 18.04][u1804], [Ubuntu 18.10][u1810], [Ubuntu 19.04][u1904],
- [Debian 9][deb9], [CentOS 7][cos], [Red Hat Enterprise Linux (RHEL) 7][rhel7],
+ [Debian 8][deb8],  [Debian 9][deb9], [CentOS 7][cos], [Red Hat Enterprise Linux (RHEL) 7][rhel7],
  [openSUSE 42.3][opensuse], [openSUSE Leap 15][opensuse], [Fedora 27][fedora], [Fedora 28][fedora],
  and [Arch Linux][arch].
 
@@ -23,6 +23,7 @@ All packages are available on our GitHub [releases][] page. After the package is
 [u1804]: #ubuntu-1804
 [u1810]: #ubuntu-1810
 [u1904]: #ubuntu-1904
+[deb8]: #debian-8
 [deb9]: #debian-9
 [cos]: #centos-7
 [rhel7]: #red-hat-enterprise-linux-rhel-7