Shorten Fedora dnf install command (#1790)

From the dnf docs:

    --repo=<repoid>, --repoid=<repoid>

Enable just specific repositories by an id or a glob. Can be used
multiple times with accumulative effect. It is basically a shortcut
for --disablerepo="*" --enablerepo=<repoid> and is mutually
exclusive with the --disablerepo option.

Author: leifmetcalf

install.markdown

@@ -42,7 +42,7 @@ Below we list steps for installing Elixir in different distributions. If your di
     * Run: `pacman -S elixir`
 
   - **Fedora**
-    * Fedora's Rawhide repository keeps more recent versions: `sudo dnf --disablerepo='*' --enablerepo=rawhide install elixir elixir-doc erlang erlang-doc`
+    * Fedora's Rawhide repository keeps more recent versions: `sudo dnf --repo=rawhide install elixir elixir-doc erlang erlang-doc`
     * You may use the default distribution, but those often lag behind: `sudo dnf install elixir erlang`
     * Documentation is available in separate packages: `sudo dnf install elixir-doc erlang-doc`