Fixes #56: Typo in readme

This is a pretty minor change.

The readme should say that knife commands are not support, but should say supported: 

```NOTE: chef-vault 1.0 knife commands are not support! Please use chef-vault 2.0 commands.```

but should say

```NOTE: chef-vault 1.0 knife commands are not supported! Please use chef-vault 2.0 commands.```