fix: add missing cookbook recipe snippet imports (#853)

* fix: add missing cookbook recipe snippet imports

* Update docs/cookbook.rst

---------

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Author: KasperZutterman

docs/cookbook.rst

@@ -40,6 +40,7 @@ Enter the ``dev`` nox session:
 .. code-block:: python
 
     import os
+    import pathlib
 
     import nox
 
@@ -87,6 +88,7 @@ Even more so with a sprinkling of Nox:
 
 .. code-block:: python
 
+    import argparse
     import nox
 
     @nox.session