Fix error when using `npm help foo bar baz`.

Fixes the following exception:

4 error ReferenceError: num is not defined
4 error     at EventEmitter.help (/usr/local/lib/node_modules/npm/lib/help.js:27:46)
4 error     at Object.commandCache.(anonymous function) (/usr/local/lib/node_modules/npm/lib/npm.js:193:11)
4 error     at EventEmitter.<anonymous> (/usr/local/lib/node_modules/npm/bin/npm-cli.js:83:28)
4 error     at process._tickCallback (node.js:415:13)