[fractal] Correct rust-stable extension version in README.md

[Alejandro Domínguez <aledomu src gnome org>]

commit 59d202e0826a7f5a6bee18271d11895bc197eba9
Author: Francesco Mazzoli <f mazzo li>
Date:   Mon Jan 11 14:46:32 2021 +0000

    Correct rust-stable extension version in README.md

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index fc900e34..d36858f0 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,7 @@ flatpak remote-add --user --if-not-exists gnome-nightly https://nightly.gnome.or
 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//19.08
+flatpak install --user flathub org.freedesktop.Sdk.Extension.rust-stable//20.08
 ```
 
 Then you go ahead and build Fractal.