Update URL for docbook.xsl

[russellshackleford]

Per https://lists.oasis-open.org/archives/docbook-apps/201612/msg00011.html Docbook XSL stylesheets are no longer hosted at sourceforge.

[mattbishop]

+1 This should be an easy one to merge, and it breaks the build for git-crypt on MacPorts and probably Brew.

[eggbean]

@mattbishop The file still seems to be hosted at the sourceforge link, so I don't understand why I was still getting this error.

error : Unknown IO error
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
make: *** [Makefile:58: man/man1/git-crypt.1] Error 4
Error: Process completed with exit code 2.

However, the issue was resolved by installing docbook-xsl before building on Ubuntu.