fix typo in extrapackages example (#10835)

Co-authored-by: Jean-François B <2589111+jfbu@users.noreply.github.com>

doc/latex.rst

@@ -233,7 +233,7 @@ Keys that you may want to override include:
    .. code-block:: python
 
        latex_elements = {
-           'packages': r'\usepackage{isodate}'
+           'extrapackages': r'\usepackage{isodate}'
        }
 
    The specified LaTeX packages will be loaded before