Address warning about unknown option

This would otherwise fail the spellcheck during CI.

api.rst

@@ -918,7 +918,7 @@ File formats and translation functions
    and the translated text marked with the keyword :token:`msgstr`.  A
    :file:`po` file that contains all needed :token:`msgid`'s can be
    created e.g. by the gnu gettext tool :program:`xgettext` with the
-   option :option:`-k SANE_I18N`.  The translator adds the translated
+   option ``-k SANE_I18N``.  The translator adds the translated
    texts to the :file:`po` files.  The gettext tool :program:`msgfmt`
    converts the :file:`po` files to the :file:`mo` or :file:`gmo`
    files.