doc: Fix "Prev" link.

Presently, the "Prev" link on the page for background workers sends
you to the middle of the previous chapter instead of the actual
previous page.  This appears to be caused by a libxml2 bug, but
regardless, a minimal fix is to change the link generation code to
use [position()=last()] instead of [last()] in the predicate on the
union of reverse axes.

Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/aim4AZorFKaC7Wrf%40nathan
Backpatch-through: 14

doc/src/sgml/stylesheet-speedup-xhtml.xsl

@@ -208,7 +208,7 @@
              |ancestor::article[1]
              |ancestor::topic[1]
              |preceding::sect1[1]
-             |ancestor::sect1[1])[last()]"/>
+             |ancestor::sect1[1])[position()=last()]"/>
 
   <xsl:variable name="next"
     select="(following::book[1]