doc: specify that errno is a number, not a string

The documentation erroneously described the errno property as an alias
for the code property, but that is not the case in the implementation.
errno is the error code of the error as a number, and code is the error
code of the error as a string.

PR-URL: https://github.com/nodejs/node/pull/9007


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>