Fix tld_length documentation in ActionDispatch::Cookies

[mltsy]

Summary

Change recommendation for tld_length (for sharing cookies across subdomains of a 2-token TLD), to 2 instead of 1 (TLD isn't exactly the right name here, but since that's what the option is called, that's what I'm calling it). This option actually specifies the number of tokens in the root domain across whose subdomains you want to share the cookie. In the example given, it is 2 (lvh and me), so recommending 1 is confusing. (calling it tld_length is also confusing, but c'est la vie) 😉

With a setting of 2, the domain for the cookie is set to: .lvh.me which is the desired result.

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[mltsy]

I suppose the rest of the text for this description could probably be updated too, since it seems to indicate that tld_length is for setting the length of the TLD. Alternately, if this option is actually supposed set the length of the TLD (rather than the root domain for cookie sharing), and domain: :all is supposed to add one more token to that TLD to determine the root domain for cookie sharing, then the implementation is incorrect... not sure which is the case!

I'd be happy to update this PR in either case, given direction :)