ffprobe: fix format section XML validation

It was broken since 2186a7e547.

doc/ffprobe.xsd

@@ -178,6 +178,7 @@
 
       <xsd:attribute name="filename"         type="xsd:string" use="required"/>
       <xsd:attribute name="nb_streams"       type="xsd:int"    use="required"/>
+      <xsd:attribute name="nb_programs"      type="xsd:int"    use="required"/>
       <xsd:attribute name="format_name"      type="xsd:string" use="required"/>
       <xsd:attribute name="format_long_name" type="xsd:string"/>
       <xsd:attribute name="start_time"       type="xsd:float"/>