update example on the front docs page

Author: mfeurer

doc/index.rst

@@ -37,7 +37,7 @@ Example
     # Download the OpenML task for the german credit card dataset with 10-fold
     # cross-validation.
     task = openml.tasks.get_task(31)
-    # Run the scikit-learn model on the task (requires an API key).
+    # Run the scikit-learn model on the task.
     run = openml.runs.run_model_on_task(clf, task)
     # Publish the experiment on OpenML (optional, requires an API key).
     run.publish()

doc/usage.rst

@@ -8,9 +8,9 @@
 .. role:: python(code)
    :language: python
 
-***********
-Basic Usage
-***********
+**********
+User Guide
+**********
 
 This document will guide you through the most important use cases, functions
 and classes in the OpenML Python API. Throughout this document, we will use