[Doc] Modernize Class name in examples

[smnandre]

From "Project_Twig_Extension" to "CustomExtension"

Just a suggestion, feel free to close if you want to keep it like this :)

I found those namings only in "advanced"

[fabpot]

That makes a lot of sense to me. Have you change them all?

[smnandre]

That makes a lot of sense to me. Have you change them all?

I looked for other Foo_Bar_Class and could not find one.. it seems the remaining were mostly in this advanced page.

But i noticed something "similar-ish" a couple weeks ago (and forgot to note it...) i'll look if i can find it again

[smnandre]

Got it :) https://github.com/twigphp/Twig/pull/4117

[fabpot]

Thank you @smnandre.