Update padding.rst
Small typo fix
diff --git a/docs/hazmat/primitives/padding.rst b/docs/hazmat/primitives/padding.rst
index 4d79ac8..764fef4 100644
--- a/docs/hazmat/primitives/padding.rst
+++ b/docs/hazmat/primitives/padding.rst
@@ -54,7 +54,7 @@
.. class:: PaddingContext
- When calling ``padder()`` or ``unpadder()`` you will receive an a return
+ When calling ``padder()`` or ``unpadder()`` you will receive a return
object conforming to the ``PaddingContext`` interface. You can then call
``update(data)`` with data until you have fed everything into the context.
Once that is done call ``finalize()`` to finish the operation and obtain