[fractal] Fix Rust version in README

[Daniel Garcia Moreno <danigm src gnome org>]

commit f99f14932c1e5297477e96eb00df785a98b4f8a4
Author: Florian Anderiasch <fa art-core org>
Date:   Mon Sep 23 19:10:47 2019 +0200

    Fix Rust version in README

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 41e030b2..f4cf0bd4 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ flatpak remote-add --user --if-not-exists gnome-nightly https://sdk.gnome.org/gn
 flatpak install --user gnome-nightly org.gnome.Sdk org.gnome.Platform
 
 # Install the required rust-stable extension from Flathub
-flatpak install --user flathub org.freedesktop.Sdk.Extension.rust-stable//18.08
+flatpak install --user flathub org.freedesktop.Sdk.Extension.rust-stable//19.08
 ```
 
 Then you go ahead and build Fractal.