[fractal/sdk-version] Bump Rust extension version in instructions

[Alexandre Franke <afranke src gnome org>]

commit 3eb5392f3ceedbe5aa67209c1df638f1aa906b4f
Author: Alexandre Franke <afranke gnome org>
Date:   Sun Sep 26 13:14:04 2021 +0200

    Bump Rust extension version in instructions

 CONTRIBUTING.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 97425020..f8b7efe2 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -9,7 +9,7 @@ We only support building with [flatpak](https://flatpak.org/) using [GNOME Build
 If you insist on using any other method, they may work, but you will be on your own.
 
 You will need to manually install the latest version of the Rust extension of the FreeDesktop SDK.
-To do this, just run `flatpak install --user org.freedesktop.Sdk.Extension.rust-stable//20.08`
+To do this, just run `flatpak install --user org.freedesktop.Sdk.Extension.rust-stable//21.08`
 
 ## Merge requests