Correct URL to API reference

DK why nor when, but it seems URL to API reference changed : now with a php extension instead of html
Correct URL to API reference

Same as https://github.com/jelix/docs-jelix-org/pull/14 but for english template (sorry for separate PR, but I'm using GitHub editor for convinience ...)
Merge branch 'patch-2' of git://github.com/bricet/docs-jelix-org