setup: fix extras_require name

a365cf0bc2ea ("setup: install matplotlib and ipython only if we are
going to use it with notebooks") added the ability to install more
python packages if the user was interested in running trappy in an
ipython notebook.  However, extras_require was mispelled.  Fix it.
diff --git a/setup.py b/setup.py
index c9c0003..43d1eef 100644
--- a/setup.py
+++ b/setup.py
@@ -69,5 +69,5 @@
           "Topic :: Scientific/Engineering :: Visualization"
       ],
       install_requires=REQUIRES,
-      extras_requires=EXTRAS
+      extras_require=EXTRAS
       )