Fix a broken link to W3Schools HTML5 reference

[CelieArnaud]

Fix for versions 4.0, 4.1, 4.2 and 5.0 of tuto

[balat]

This should be done also in repository tuto.
ocsigen.org-data contains a copy of the doc (which was done automatically by jenkins until recently).
If you change something here only this could be overwritten at next update.
(That's one of the reason I want to get rid of ocsigen.org-data)