news: Fix version number

The schema expects it to match the pattern

  v[0-9]+\.[0-9]+\.[0-9]+

which "5.0.0" clearly doesn't, causing the build to fail.
Reported-by: NPeter Krempa <pkrempa@redhat.com>
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

docs/news.xml

@@ -33,7 +33,7 @@
      -->
 
 <libvirt>
-  <release version="5.0.0" date="unreleased">
+  <release version="v5.0.0" date="unreleased">
     <section title="New features">
     </section>
     <section title="Improvements">