Spec header should ask file to issue at github

[kojiishi]

From @bzbarsky, from a www-style thread:

Does the writing-modes spec really have no way to file issues other than the mailing list? Certainly the header claims so. :(

Is this something we can fix in bikeshed?

[kojiishi]

@tabatkins

[bzbarsky]

@dbaron fixed this already, after that thread.

[dbaron]

Fixed in:

• https://hg.csswg.org/drafts/rev/e7064dca3c6f
• https://hg.csswg.org/drafts/rev/2fe47b543fd2
• https://hg.csswg.org/drafts/rev/6204d8b73221
• https://hg.csswg.org/drafts/rev/3576c5f95707