Fix a couple links in documentation

Also upgrade nbsphinx version.

docs/api_ref.rst

@@ -7,6 +7,6 @@ functions in the ``stix2`` API, as given by the package's docstrings.
 .. note::
     All the classes and functions detailed in the pages below are importable
     directly from `stix2`. See also:
-    :ref:`How imports will work <guide/ts_support.ipynb#How-imports-will-work>`.
+    :ref:`How imports will work </guide/ts_support.ipynb#How-imports-will-work>`.
 
 .. automodule:: stix2

docs/guide/creating.ipynb

@@ -381,7 +381,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "To update the properties of an object, see the [Versioning](versioning.ipynb) section."
+    "To update the properties of an object, see the [Versioning](guide/versioning.ipynb) section."
    ]
   },
   {

requirements.txt

@@ -1,5 +1,6 @@
 bumpversion
-nbsphinx>=0.2.15
+ipython
+nbsphinx>=0.3.0
 pre-commit
 pytest
 pytest-cov