Use 'utf-8' instead of 'utf8' to be standard compliant.

Using 'utf8' is not compatible with some IE versions like IE9 (loading page throw a script error “c00ce56e”).

see this article on [invalid charset](http://blogs.msdn.com/b/ieinternals/archive/2011/02/28/script-error-c00ce56e-means-you-have-specified-an-invalid-charset-utf8-is-not-utf-8.aspx?Redirected=true)

also see section "Official name and variants" in [wikipedia](https://en.wikipedia.org/wiki/UTF-8)

> Other descriptions that omit the hyphen or replace it with a space, such as "utf8" or "UTF 8", are not accepted as correct by the governing standards.

note: in _html5boilerplate_ the charset is defined as `utf-8`