fix into README.md
Hi, in README.md, link to readthedocs should be fixed from https://tango-controls.readthedocs.io/en/latest/tools-and-extensions/astor/index.html?highlight=Astor to https://tango-controls.readthedocs.io/en/latest/tools-and-extensions/built-in/astor/index.html?highlight=Astor