Change variable name to fix zsh parse issue

Using 'A' as a variable throws the following error using zsh (oh-my-zsh) on OSX 10.9.1 when trying to source `nvm.sh`

```sh
    $HOME/.nvm/nvm.sh:367: parse error near `|'
```
Not sure why, but changing the variable name to anything _but_ 'A' seems to fix the parse error.