README: tweak `cosa build` description
Remove the reference to `repo/`; that's no longer used. Also, `cosa
build` only builds a the qemu image.
@@ -195,8 +195,8 @@ And now we can build from these inputs:
$ cosa build
```
-Each build will write an OSTree commit into `$PWD/repo/` as well
-as generate VM images in `$PWD/builds/`.
+Each build will create a new directory in `$PWD/builds/`, containing the
+generated OSTree commit (as a tarball) and the qemu VM image.
Next, rerun `cosa build` and notice the system correctly
deduces that nothing changed. You can run `cosa fetch`
