Fix license string

Author: Steve Ayers

pyproject.toml

@@ -6,7 +6,7 @@ build-backend = "hatchling.build"
 name = "protovalidate"
 description = "Protocol Buffer Validation for Python"
 readme = "README.md"
-license = "Apache 2.0"
+license = "Apache-2.0"
 license-files = ["LICENSE"]
 keywords = ["validate", "protobuf", "protocol buffer"]
 requires-python = ">=3.9"