org-html-link-org-files-as-html

org-html-link-org-files-as-html is a variable defined in `ox-html.el'.
Its value is

t

Documentation:
Non-nil means make file links to `file.org' point to `file.html'.
When `org-mode' is exporting an `org-mode' file to HTML, links to
non-html files are directly put into a href tag in HTML.
However, links to other Org-mode files (recognized by the
extension `.org.) should become links to the corresponding html
file, assuming that the linked `org-mode' file will also be
converted to HTML.
When nil, the links still point to the plain `.org' file.

You can customize this variable.