docs: fix references to docs/devel/atomics.rst

docs: fix references to docs/devel/atomics.rst

Commit 15e8699f00 ("atomics: convert to reStructuredText") converted
docs/devel/atomics.txt to docs/devel/atomics.rst.

We still have several references to the old file, so let's fix them
with the following command:

  sed -i s/atomics.txt/atomics.rst/ $(git grep -l docs/devel/atomics.txt)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210517151702.109066-3-sgarzare@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>