Use sdk version 22.08

I just made a PR to the Flathub and I got the feedback that I should use version 22.08 of the SDK instead of 21.08, so I thought it might be good to have that version in the getting started documentation as well.

docs/first-build.rst

@@ -19,10 +19,10 @@ dependencies. Each runtime has a matching SDK (Software Development Kit), which
 contains all the things that are in the runtime, plus headers and development
 tools. This SDK is required to build apps for the runtime.
 
-In this tutorial we will use the Freedesktop 21.08 runtime and SDK. To
+In this tutorial we will use the Freedesktop 22.08 runtime and SDK. To
 install these, run::
 
-  $ flatpak install flathub org.freedesktop.Platform//21.08 org.freedesktop.Sdk//21.08
+  $ flatpak install flathub org.freedesktop.Platform//22.08 org.freedesktop.Sdk//22.08
 
 2. Create the app
 -----------------
@@ -46,7 +46,7 @@ manifest to the hello world app, add the following to an empty file:
 
   app-id: org.flatpak.Hello
   runtime: org.freedesktop.Platform
-  runtime-version: '21.08'
+  runtime-version: '22.08'
   sdk: org.freedesktop.Sdk
   command: hello.sh
   modules: