Correct `npm cache clean` command in the README

Change `npm cache clear` -> `npm cache clean` command in [Motivation / Reduce diff churn](https://github.com/uber/npm-shrinkwrap#reduce-diff-churn) section of the README. 

I'm assuming this is a typo as `npm cache clean` is listed as a command in the [npm-cache](https://www.npmjs.org/doc/cli/npm-cache.html) docs, but not `npm cache clear`.