fix: fix extra_require typo (#202)

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

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Fixes #203  🦕

packages/google-cloud-containeranalysis/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,