fix: shift 2 when consuming 2 arguments

[pmalek]

When using --indent we should shift 2 arguments instead of 1 to avoid the error as shown below.

$ gh-md-toc --indent 2 --no-backup README.md



rm: illegal option -- -
usage: rm [-f | -i] [-dIPRrvWx] file ...
       unlink [--] file
Parsing local markdown file requires access to github API
Please make sure curl is installed and check your network connectivity

[ekalinin]

Fixed in #132

[ekalinin]

Thanks, anyway!