Update submodule.js

In getSubmodules "args" was defined as "arg", which caused the following error:
```
npm ERR! ReferenceError: args is not defined
npm ERR!     at /usr/share/npm/lib/submodule.js:102:15
npm ERR!     at /usr/share/npm/node_modules/which/which.js:55:18
npm ERR!     at Object.oncomplete (fs.js:107:15)
npm ERR! If you need help, you may report this log at:
npm ERR!     <http://github.com/isaacs/npm/issues>
npm ERR! or email it to:
npm ERR!     <npm-@googlegroups.com>
```