doc: fix the exception description

A value shouldn't be described as doing inherit from some class, more
strictly, the value is an instance of the class `Error`.

PR-URL: https://github.com/nodejs/node/pull/3658


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Stephan Belanger <admin@stephenbelanger.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>