fix code emphasis which fixes a build warning

Author: bast

content/packaging.rst

@@ -107,7 +107,7 @@ This is how ``pyproject.toml`` looks:
 
 .. literalinclude:: packaging-example-project/pyproject.toml
    :caption: pyproject.toml
-   :emphasize-lines: 18-20
+   :emphasize-lines: 13-15
 
 Note how our package requires ``scipy`` and we decided to not pin the version
 here (see :ref:`version_pinning`).