Fix typo in setuptools extras declaration (#2100)

The name of the key is extras_require not extra_requires to declare
optional dependencies. [1] This commit corrects the typo so it works
correctly.

[1] https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

setup.py

@@ -93,7 +93,7 @@ setup(
     package_data=PACKAGE_DATA,
     include_package_data=True,
     python_requires=">=3.5",
-    extra_requires={
+    extras_require={
         'visualization': ['matplotlib>=2.1', 'nxpd>=0.2', 'ipywidgets>=7.3.0',
                           'pydot'],
         'full-featured-simulators': ['qiskit-aer>=0.1']