Update readme for index.rst

Author: Jessica Lin

README.md

@@ -15,7 +15,7 @@ Here's how to create a new tutorial:
 1. Create a notebook styled python file. If you want it executed while inserted into documentation, save the file with suffix `tutorial` so that file name is `your_tutorial.py`.
 2. Put it in one of the beginner_source, intermediate_source, advanced_source based on the level.
 2. Include it in the right TOC tree at index.rst
-3. Create a thumbnail in the index file using a command like `.. galleryitem:: beginner/your_tutorial.py`. (This is a custom directive. See `custom_directives.py` for more info.) 
+3. Create a thumbnail in the index.rst file using a command like `.. customcarditem:: beginner/your_tutorial.html`. (This is a custom directive. See `custom_directives.py` for more info.) 
 
 In case you prefer to write your tutorial in jupyter, you can use [this script](https://gist.github.com/chsasank/7218ca16f8d022e02a9c0deb94a310fe) to convert the notebook to python file. After conversion and addition to the project, please make sure the sections headings etc are in logical order.