Correct options for space-index-pages-free-plot

Previously this failed with:

bin/innodb_space:794:in `basename': can't convert nil into String (TypeError)
        from bin/innodb_space:794:in `block in <main>'
        from bin/innodb_space:760:in `each'
        from bin/innodb_space:760:in `<main>'
Merge pull request #8 from maginatics/space-index-pages-free-plot-space-file

Correct options for space-index-pages-free-plot