doc: Clean up warnings when building notmuch-emacs docs

doc: Clean up warnings when building notmuch-emacs docs

Without this change, we see this during the build:

    sphinx-build -b html -d doc/_build/doctrees -q ./doc doc/_build/html
    …/doc/notmuch-emacs.rst:67: WARNING: Unexpected indentation.
    …/doc/notmuch-emacs.rst:165: WARNING: Unexpected indentation.
    …/doc/notmuch-emacs.rst:306: WARNING: Unexpected indentation.

This source change doesn't seem to have any effect on the generated
HTML, at least.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>