Update README.md for pdf print instructions

[Zacaria]

Hi !

I was trying to print my presentation, but I found out the documentation is outdated.

Thank you for your tool !

[webpro]

Thanks, @Zacaria!

Are you on the latest version of reveal-md? The docs were updated to match the Reveal.js v5 update.

[Zacaria]

Oh, it's true that I didn't see the changelog.

However I just checked again, on my project : https://github.com/Zacaria/apidays-2023 I have version 5.5.2

?view=print gives me this result

?print-pdf gives me this result

Then I believe it's more an issue than a missing documentation

I leave it up to you to close this PR :)

Do you want me to try and fix it ?

[webpro]

Do you want me to try and fix it ?

I would love that! Personally I'm not using the print/PDF functionality at all.