Fix name of function under test.

Prevent a 'lein midje' run from failing with:
  Exception in thread "main" java.lang.RuntimeException:
    Unable to resolve symbol: nginx-log-entries-from in this context,
    compiling:(lumberjack/nginx_test.clj:52:12)
Probably caused by renaming the function under test in the source code but not in the test code.
(Keep on going - I'm eager to see how this interesting project develops in the future!)