fix typo in extrapackges example

[king-phyte]

Subject: Fix typo in extrapackages example in LaTeX customization docs

Feature or Bugfix

• Bugfix

Purpose

The example in the docs for extrapackages under LaTeX customization is slightly incorrect (as shown below).

latex_elements = {
    'packages': r'\usepackage{isodate}'
}

Doing so will lead to Sphinx producing the following warning WARNING: Unknown configure key: latex_elements['packages'], ignored.