docs: remove out-of-date bug warning in Reponsive Navigation
docs: remove out-of-date bug warning in Reponsive Navigation

Introduced in https://github.com/zurb/foundation-sites/commit/d93ac93941ae1b01165f0f97fc7f60f6679811d9 (https://github.com/zurb/foundation-sites/pull/10058)

Bug in v6.3.1: https://codepen.io/IamManchanda/pen/OmawKe
Resolved in v6.4.3: https://codepen.io/ncoden/pen/XZQBPJ

zurb/foundation-sites,jamesstoneco/foundation-sites,DaSchTour/foundation-sites,atmmarketing/foundation-sites,DaSchTour/foundation-sites,jamesstoneco/foundation-sites,atmmarketing/foundation-sites,abdullahsalem/foundation-sites,abdullahsalem/foundation-sites,abdullahsalem/foundation-sites,zurb/foundation,zurb/foundation,colin-marshall/foundation-sites,denisahac/foundation-sites,zurb/foundation-sites,zurb/foundation-sites,denisahac/foundation-sites,colin-marshall/foundation-sites,colin-marshall/foundation-sites,zurb/foundation,atmmarketing/foundation-sites,jamesstoneco/foundation-sites,denisahac/foundation-sites