Fix for doc reference to python "raise" statement

:ref:`py:raise` -> :ref:`raise statement <py:raise>`

Before fix the text reads: "...define an on_error handler consisting
of a single empty The raise statement."　After fix it should read: 
"...define an on_error handler consisting of a single empty raise
statement."

Author: ChasL

docs/api.rst

@@ -267,7 +267,7 @@ to handle it, which defaults to print a traceback and ignoring the exception.
 
     If you want exception to propagate out of the :class:`Client` class
     you can define an ``on_error`` handler consisting of a single empty
-    :ref:`py:raise`.  Exceptions raised by ``on_error`` will not be
+    :ref:`raise statement <py:raise>`. Exceptions raised by ``on_error`` will not be
     handled in any way by :class:`Client`.
 
     .. note::