Use valid licence registry identifier for Apache-2.0 licence

This is so that Poetry sets the licence classifier correctly.
It only parses licences that contain a precise licence registry identifier

Author: Veronika Siska

pyproject.toml

@@ -14,7 +14,7 @@ name = "openapi-spec-validator"
 version = "0.3.2"
 description = "OpenAPI 2.0 (aka Swagger) and OpenAPI 3.0 spec validator"
 authors = ["Artur Maciag <maciag.artur@gmail.com>"]
-license = "Apache License, Version 2.0"
+license = "Apache-2.0"
 readme = "README.rst"
 repository = "https://github.com/p1c2u/openapi-spec-validator"
 keywords = ["openapi", "swagger", "schema"]