use --force-clean in Step 5

When following the tutorial, we need to use `--force-clean` in Step 5 (Test the build) to avoid a `App dir 'build-dir' is not empty`

docs/first-build.rst

@@ -82,9 +82,13 @@ it to the ``/app`` subdirectory, inside the ``build-dir`` directory.
 
 To verify that the build was successful, run the following::
 
-  $ flatpak-builder --user --install build-dir org.flatpak.Hello.yml
+  $ flatpak-builder --user --install --force-clean build-dir org.flatpak.Hello.yml
   $ flatpak run org.flatpak.Hello
 
+This second time we passed in ``--force-clean``, which means that the
+previously created ``build-dir`` directory was deleted before the new build was
+started.
+
 Congratulations, you've made an app!
 
 6. Put the app in a repository
@@ -101,10 +105,6 @@ directory called ``repo``. Note that ``flatpak-builder`` keeps a cache of
 previous builds in the ``.flatpak-builder`` subdirectory, so doing a second
 build like this is very fast.
 
-This second time we passed in ``--force-clean``, which means that the
-previously created ``build-dir`` directory was deleted before the new build was
-started.
-
 In order for your application to show up in application stores while testing with a local repository, you might have
 to run ``flatpak build-update-repo repo``.
 For more information how to publish to application stores see `Appdata files <http://docs.flatpak.org/en/latest/freedesktop-quick-reference.html#appdata-files>`_.