Fix boo-boo in prep_tarball script.

A hardcoded a branch number was in the prep_tarball which could not work. Changed
it to the  variable.
........

Merged revisions 343789 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@343790 65c4cc65-6c06-0410-ace0-fbb531ad65f3

build_tools/prep_tarball

@@ -24,6 +24,6 @@ wget https://wiki.asterisk.org/wiki/download/attachments/19005471/Asterisk-Admin
 echo "Extracting HTML Admin Guide"
 unzip Asterisk-Admin-Guide-$branch.html.zip
 mv AST/ Asterisk-Admin-Guide/
-mv Asterisk-Admin-Guide-1.8.pdf Asterisk-Admin-Guide.pdf
+mv Asterisk-Admin-Guide-$branch.pdf Asterisk-Admin-Guide.pdf
 rm -f Asterisk-Admin-Guide-$branch.html.zip
 echo "Documentation downloaded. Goodbye!"