fix Sphinx warnings

README.rst

adaptive/learner/base_learner.py

adaptive/learner/skopt_learner.py

adaptive/runner.py

docs/source/conf.py

docs/source/reference/adaptive.learner.base_learner.rst

docs/source/reference/adaptive.rst

docs/source/reference/adaptive.runner.extras.rst

docs/source/tutorial/tutorial.custom_loss.rst

docs/source/tutorial/tutorial.rst