Avoid trailing slash on void elements.

[zapotoczky]

W3C throws info warnings on trailing slashes on void elements: https://github.com/validator/validator/wiki/Markup-%C2%BB-Void-elements#trailing-slashes-in-void-element-start-tags-do-not-mark-the-start-tags-as-self-closing and https://github.com/validator/validator/wiki/Markup-%C2%BB-Void-elements#trailing-slashes-directly-preceded-by-unquoted-attribute-values Would be cool, if they could be not rendered by react-helmet-async