Clarified docs

python_examples/train_object_detector.py

 #!/usr/bin/python
 # The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
 #
-# This example program shows how you can use dlib to make an object
+# This example program shows how you can use dlib to make a HOG based object
 # detector for things like faces, pedestrians, and any other semi-rigid
 # object.  In particular, we go though the steps to train the kind of sliding
 # window object detector first published by Dalal and Triggs in 2005 in the