Remove types from the docstring
They are redundant with types in the signature, and make the documentation build fail with:
/home/jenkins/workspace/DDOC/dev/swh-docs/.tox/sphinx-dev/lib/python3.7/site-packages/swh/spdx/traverse.py:docstring of swh.spdx.traverse.traverse_root:1: WARNING: more than one target found for cross-reference 'Node': swh.graph.grpc.swhgraph_pb2.Node, swh.spdx.node.Node