[geary] Mention dnf in INSTALL for Fedora, not yum.

[Michael Gratton <mjog src gnome org>]

commit bac17b720b10a8c894fb6c84ae6d9832f80c87fb
Author: Michael James Gratton <mike vee net>
Date:   Thu Feb 2 00:48:24 2017 +1100

    Mention dnf in INSTALL for Fedora, not yum.

 INSTALL |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/INSTALL b/INSTALL
index 86cdc70..e162ad1 100644
--- a/INSTALL
+++ b/INSTALL
@@ -65,7 +65,7 @@
     Fedora 23 and later ships with the correct versions of the
     required libraries. Install them by running this command:
 
-        $ sudo yum install vala gobject-introspection-devel intltool cmake \
+        $ sudo dnf install vala gobject-introspection-devel intltool cmake \
             desktop-file-utils gnome-doc-utils libcanberra-devel libgee-devel \
             glib2-devel gmime-devel gtk3-devel libnotify-devel sqlite-devel \
             unique3-devel webkitgtk4-devel libsecret-devel libxml2-devel \