They are the same thing. Spell it "template." "Templet" is a very rare spelling.
I am surprised to find that it really is in the American Heritage Dictionary as an "alternative to 'template.'" However, I'm a U.S. speaker and I have never, ever, EVER seen it spelled that way until I read this question.
When I do a Google search on "templet" it asks "Do you mean template?" and reports 3,430,000 results for "templet." It reports 850,000,000 results for "template."