Replace cause method to property usage in README

[ivanmartynau-oss]

Summary

• The README showed err.cause() as an instance method, which does not exist and throws TypeError.
• cause on a VError instance is a property (err.cause). The static helper remains VError.cause(err).
• Update the quick-start example to err.cause.message so it matches lib/verror.js.

const err = new VError(err1, 'stat "%s"', filename);

err.cause                    // the Error object (or undefined if none)
VError.cause(err)            // same Error, or null if none
err.cause()                 // TypeError