org-html-footnotes-section

org-html-footnotes-section is a variable defined in `ox-html.el'.
Its value is

"
\n
%s: 
\n
\n%s\n
\n
"

Documentation:
Format for the footnotes section.
Should contain a two instances of %s. The first will be replaced with the
language-specific word for "Footnotes", the second one will be replaced
by the footnotes themselves.

You can customize this variable.