setup.py: Fix extra_require typo

This allows someone to install the module with the `[libcst]` extra.

Author: major

setup.py

@@ -82,7 +82,7 @@
     packages=packages,
     namespace_packages=namespaces,
     install_requires=dependencies,
-    extras_requires=extras,
+    extras_require=extras,
     python_requires=">=3.6",
     scripts=["scripts/fixup_containeranalysis_v1_keywords.py"],
     include_package_data=True,