fix link to libarchive windows installer

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| Maruku tells you:
+---------------------------------------------------------------------------
| Could not find ref_id = "httpdownloadssourceforgenetgnuwin32libarchive24121setupexe" for md_link([
|   "http://downloads.sourceforge.net/gnuwin32/libarchive-2.4.12-1-setup.exe"
| ],"httpdownloadssourceforgenetgnuwin32libarchive24121setupexe")
| Available refs are ["wikigzip", "wikitar", "wikiark", "wikifileroller", "winrar", "7zip", "libarchive", "libarchive2", "libarchivetool", "winsyspath"]
+---------------------------------------------------------------------------
!/usr/lib/ruby/gems/1.9.1/gems/maruku-0.6.1/lib/maruku/errors_management.rb:49:in `maruku_error'
!/usr/lib/ruby/gems/1.9.1/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:716:in `to_html_link'
!/usr/lib/ruby/gems/1.9.1/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:970:in `block in array_to_html'
!/usr/lib/ruby/gems/1.9.1/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:961:in `each'
!/usr/lib/ruby/gems/1.9.1/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:961:in `array_to_html'
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Not creating a link for ref_id = "httpdownloadssourceforgenetgnuwin32libarchive24121setupexe".