Fix rust extension version in flatpak manifest

Author: ranfdev

build-aux/com.ranfdev.Geopard.json

@@ -4,7 +4,7 @@
     "runtime-version" : "42",
     "sdk" : "org.gnome.Sdk",
     "sdk-extensions" : [
-        "org.freedesktop.Sdk.Extension.rust-stable//21.08"
+        "org.freedesktop.Sdk.Extension.rust-stable"
     ],
     "command" : "geopard",
     "tags" : [