Use correct variable

Fixes:

    Error: /usr/local/share/lua/5.1/lapis/cmd/nginx.lua:45: attempt to index global 'url' (a nil value)
    stack traceback:
        /usr/local/share/lua/5.1/lapis/cmd/nginx.lua:45: in function </usr/local/share/lua/5.1/lapis/cmd/nginx.lua:38>
        (tail call): ?
        [C]: in function 'gsub'
        /usr/local/share/lua/5.1/lapis/cmd/nginx.lua:83: in function 'compile_config'
        /usr/local/share/lua/5.1/lapis/cmd/nginx.lua:118: in function 'write_config_for'
        /usr/local/share/lua/5.1/lapis/cmd/actions.lua:143: in function </usr/local/share/lua/5.1/lapis/cmd/actions.lua:135>
        (tail call): ?
        [C]: in function 'xpcall'
        /usr/local/share/lua/5.1/lapis/cmd/actions.lua:332: in function 'execute'
        /usr/local/lib/luarocks/rocks/lapis/dev-1/bin/lapis:4: in main chunk
        [C]: ?