Fix reference to twelve-lines to be ten-lines

```bash
$ lein midje :print-facts
= Namespace avi.core-test
Checking regarding displaying of a loaded file
Exception in thread "main" java.lang.RuntimeException: Unable to resolve symbol: twelve-lines in this context, compiling:(avi/core_test.clj:32:1)
```
Merge pull request #1 from stevenproctor/patch-1

Fix reference to twelve-lines to be ten-lines