Update config_pgtune.rb

My testing and compilations blow up on the following regex from line 111:

```
if (max_connections.match(/\A[1-9]\d*\Z/) == nil)
```

However, if you make that capital "Z" and lowercase "z", then I think this will work:

```
if (max_connections.match(/\A[1-9]\d*\z/) == nil)
```


Unfortunately, to be sure about this, I will have to fork your cookbook and make the change there, and see if that means my code can now compile.  Sigh....