Update Index - Interactive Widgets.ipynb

Fixed link for "what is the ipython notebook"

Author: marktab

Index - Interactive Widgets.ipynb

@@ -48,7 +48,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "* [What is the IPython Notebook?](examples/Notebook/What%20is%20the%20IPython%20Notebook%3F.ipynb)\n",
+    "* [What is the IPython Notebook?](examples/Notebook/What%20is%20the%20IPython%20Notebook.ipynb)\n",
     "* [Notebook Basics](examples/Notebook/Notebook Basics.ipynb)\n",
     "* [Notebook Exercises](exercises/Notebook/Notebook Exercises.ipynb)"
    ]
@@ -95,4 +95,4 @@
  "metadata": {},
  "nbformat": 4,
  "nbformat_minor": 0
-}
+}