HTML errors

[coveritytest]

The generated HTML has still a few errors, see: https://validator.w3.org/nu/?doc=https%3A%2F%2Frust-lang.github.io%2FmdBook%2Fcli%2Fclean.html

[ulivz]

As this spec say, In order to define the charset encoding of an HTML document, both of these options are valid, but only one of them must appear in the document:

<!-- This is the preferred way -->
<meta charset="UTF-8">

<!-- This is the older way, also valid -->
<meta http-equiv="content-type" content="text/html; charset=UTF-8">