Update tutorial regarding fullscreen sf::style

[GuillermoUrcera]

In the Opening and managing an SFML window tutorial sf::Style::Fullscreen is shown as a valid window style to make it fullscreen, but in the documentation, this is not a valid enum value.

Update the tutorial to reflect that style no longer has fullscreen and it's now necessary to use the state.

[eXpl0it3r]

Fixed with 1e14c2b

Thank for the report!