XSLT stylesheet: Add slash to directory name

Some versions of the XSLT stylesheets don't handle the missing slash
correctly (they concatenate directory and file name without the slash).
This might never have worked correctly.

doc/src/sgml/stylesheet.xsl

@@ -9,7 +9,7 @@
 
 
 <!-- Parameters -->
-<xsl:param name="base.dir" select="'html'"></xsl:param>
+<xsl:param name="base.dir" select="'html/'"></xsl:param>
 <xsl:param name="html.stylesheet" select="'stylesheet.css'"></xsl:param>
 <xsl:param name="use.id.as.filename" select="'1'"></xsl:param>
 <xsl:param name="make.valid.html" select="1"></xsl:param>