Update Fedora repo in the install guide

The Fedora Copr repo link was pointing to an obsolete repository, containing stack 1.9.3, so I've updated the link to point to the newer repo.

Author: Iisus

doc/install_and_upgrade.md

@@ -141,7 +141,7 @@ Use the [generic Linux option](#linux).
 
 There is also an
 unofficial
-[Fedora Copr repo](https://copr.fedoraproject.org/coprs/petersen/stack/) which
+[Fedora Copr repo](https://copr.fedoraproject.org/coprs/petersen/stack2/) which
 can be enabled with: `sudo dnf copr enable petersen/stack`. Note that this Stack
 version may lag behind, so we recommend running `stack upgrade` after installing
 it.