Add viewBox attribute to storage page layout SVG image

Recently added SVG image for storage page layout lacks
a viewBox attribute which seems necessary to ensure propoer
rendering. Add it.

Author: Jonathan Katz
Discussion: https://postgr.es/m/ba31e0e1-4c9b-b309-70e8-8e7ac14fc87e%40postgresql.org

doc/src/sgml/images/pagelayout.svg

@@ -3,6 +3,7 @@
     xmlns='http://www.w3.org/2000/svg'
     width='610'
     height='210'
+    viewBox='0.00 0.00 610.00 210.00'
     shape-rendering='geometricPrecision'
     version='1.0'>
   <defs>