[fractal/fractal-next] Correct the rust-stable version in flatpak build instructions

[Julian Sparber <jsparber src gnome org>]

commit 65208c6981aa060d4d3271230d46c1c5ec84796e
Author: djrenren <john jrenner net>
Date:   Mon Sep 13 21:48:45 2021 +0000

    Correct the rust-stable version in flatpak build instructions

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 9c6f0e8f..732d7417 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,7 @@ 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//20.08
+flatpak install --user flathub org.freedesktop.Sdk.Extension.rust-stable//21.08
 ```
 **Inside** the `build-aux` folder run: