Added a closing tag to the let elements

[tomschluter]

It appears that the examples of the usage of the let element don't seem to work without a closing tag. Unfortunately a self closing tag doesn't seem to work either. So I have added closing tags to the let elements for now.

[3cp]

Note self-closing is xml syntax, not HTML5 syntax.

In HTML5, <any-tag /> is treated as <any-tag>, the / is ignored totally. HTML5 only defined few tags without need of closing like <input>, <br>, <hr>, <img>, <embed>, <col>, <param>, <source>, <track> and in html head <meta>, <base>, <link>. I might missed some.

When you write <input /> in HTML5, it still works because HTML5 sees it as <input>. This is the same reason why <let /> didn't work as intended.

[bigopon]

Thanks @tomschluter @huochunpeng.
Lgtm @EisenbergEffect. Sry i missed this

[3cp]

@bigopon note not only the <let> is incorrect, the <input> also get unnecessary /> ending which is bad example of HTML5. The blog post is affected too.

@tomschluter do you mind to clean up all <input> tags too in this PR?