correct short license text to SPDX standard Apache-2.0 (#223)

ala Chia-Network/chia-blockchain#19027

Author: altendky

setup.py

@@ -139,7 +139,7 @@ def build_extension(self, ext):
     author="Florin Chirica",
     author_email="[email protected]",
     description="Chia vdf verification (wraps C++)",
-    license="Apache License",
+    license="Apache-2.0",
     python_requires=">=3.8",
     long_description=open("README.md").read(),
     long_description_content_type="text/markdown",