added python practice book image

fixed image file name in json

Author: tbenjis

img/cover_python_practice_book.png

@@ -474,6 +474,6 @@
       "level":"Beginner",
       "info":"This book is prepared from the training notes of Anand Chitipothu. Anand conducts Python trainings classes on a semi-regular basis in Bangalore, India.",
       "url":"http://anandology.com/python-practice-book/index.html",
-      "cover": "img/cover_python_practice_book.jpg"
+      "cover": "img/cover_python_practice_book.png"
     }
 ]}