example correction: count(@) is not there anymore

## From examples on jmespath.org:

> current-node¶
> The current-node token can be used to represent the current node being evaluated. The current-node token is useful for functions that require the current node being evaluated as an argument. For example, the following expression creates an array containing the total number of elements in the foo object followed by the value of foo["bar"].
>
> foo[].[count(@), bar

```
$ jp.py -f vminfo.json "virtual_machines.count(@)"
unknown-function: Unknown function: count()
```