[PATCH website] downloads: replace yum with dnf for fedora instructions

[Jun Wu]

Looks good to me.

dnf has been the default package manager since Fedora 22 released 2 years
ago.

Excerpts from David Demelier's message of 2017-07-03 08:20:26 +0200:
> # HG changeset patch
> # User David Demelier <markand at malikania.fr>
> # Date 1499062737 -7200
> #      Mon Jul 03 08:18:57 2017 +0200
> # Node ID 6f3cfd6dcc28ac0d2aa35f19e48fe221293f16b7
> # Parent  4b88b3c4fc807a3d1e9a98b670e2f710796e73c1
> downloads: replace yum with dnf for fedora instructions
> 
> diff -r 4b88b3c4fc80 -r 6f3cfd6dcc28 templates/downloads/index.html
> --- a/templates/downloads/index.html    Sun May 14 21:47:27 2017 -0500
> +++ b/templates/downloads/index.html    Mon Jul 03 08:18:57 2017 +0200
> @@ -124,7 +124,7 @@
>  $ apt-get install mercurial
>  
>  # Fedora
> -$ yum install mercurial
> +$ dnf install mercurial
>  
>  # Gentoo
>  $ emerge mercurial