news: Fix XML validation

The news schema requires two digits for both month and day in the date
attribute. s/2/02/ in the day value of date to fix the following
'make check' failure

2165) Checking ../docs/news.xml against ../news.rng                     ...
libvirt: XML Util error : XML document failed to validate against schema:
Unable to validate doc against /home/jfehlig/virt/upstream/libvirt/build/../docs/schemas/../news.rng
Element release failed to validate attributes
Signed-off-by: NJim Fehlig <jfehlig@suse.com>

docs/news.xml

@@ -42,7 +42,7 @@
      -->
 
 <libvirt>
-  <release version="v5.10.0" date="2019-12-2">
+  <release version="v5.10.0" date="2019-12-02">
     <section title="New features">
       <change>
         <summary>