BLFS ordered book gen: fix for .xz Xorg packages

In Xorg pages where there are several packages, the .xsl for
separating packages assumed that the package names ended in
tar.bz2 and selected the substring before ".tar.bz2" to extract
the packagedir. This has changed recently for libX11, which is a
.tar.xz package: the fix is to just select the substring before
".tar." for the packagedir.

BLFS/xsl/make_book.xsl

@@ -580,7 +580,7 @@
         </para>
 
         <screen><userinput>packagedir=<xsl:value-of
-                    select="substring-before($tarball,'.tar.bz2')"/>
+                    select="substring-before($tarball,'.tar.')"/>
           <xsl:text>&#xA;</xsl:text>
           <xsl:value-of select="substring-before($install-instructions,
                                                  'as_root')"/>