npm-adduser update doc re email

Docs are no longer accurate; updating email address via npm adduser gives 403 forbidden on public registry.

Quoting from: https://www.npmjs.org/doc/cli/npm-adduser.html

````
The username, password, and email are read in from prompts.

You may use this command to change your email address, but not username or password.

To reset your password, go to https://www.npmjs.org/forgot
````

(amended commit to fix typo)

private registry, you mean?