Use the proper name for UTF-8 in documentation

[ilmari]

utf8 is the internal name for Perl's extended, lax UTF-8-like encoding, while UTF-8 is the stricter standard form

[exodist]

The less strict form was used because many versions of perl (generally older ones) have a segv when using this stricter version:

Perl/perl5#7533

See note in https://github.com/Test-More/test-more/blob/master/lib/Test2/Formatter/TAP.pm#L73

If we change this in the file you modified we probably want to change it all over the Test-Simple dist, including where this note is. But that said I suspect making this change will break Test-Simple on a lot of older perls. Is that breakage worth the benefit?

Maybe we make it conditional, it uses the stricter one on perls without the segv, but uses the one that does not explode when necessary?

[exodist]

oof, I just noticed that this is a doc only change. So it can probably be merged as is.