Fix the type error "path.resolved is not a function" when the first parameter is a file

[jzhn]

Currently when use diff with a file name as the first parameter, a Type Error is thrown:

$ npm-shrinkwrap diff npm-shrinkwrap.json npm-shrinkwrap.json
TypeError: path.resolved is not a function
    at main (*****\npm-shrinkwrap\bin\diff.js:128:38)

This is caused by a typo. After this fix it works fine.

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