LOGBACK-466 Documentation: Fix syntax issues with GEventEvaluator example

The [GEventEvaluator example in the documentation](http://logback.qos.ch/manual/filters.html#GEventEvaluator) has two syntax errors:
- ```=~``` should be used for a regex comparison with groovy, not ```~=```
- the ```&&``` on the line by itself is not valid groovy and causes the example to fail

I was investigating [LOGBACK-466](http://jira.qos.ch/browse/LOGBACK-466) when I found this. I think that bug should be able to be resolved.