Add empty subtitle to avoid weird artifact in HTML header

Signed-off-by: Bill Schmidt <wschmidt@linux.ibm.com>

Intrinsics_Reference/bk_main.xml

@@ -34,7 +34,7 @@
     <!-- TODO: Pick a Title for the new document -->
     <title>Power Vector Intrinsic Programming Reference</title>
     <!-- TODO: Either add a subtitle or remove the following line -->
-    <!-- subtitle></subtitle -->
+    <subtitle> </subtitle>
 
     <info>
     <author>