manual: Use Unicode instead HTML entities for characters (bug 19737)

Texinfo no longer treats arguments to @set in @ifhtml blocks as
literal HTML, so the & in the entity references was encoded as
@amp; in HTML.  Using the equivalent Unicode characters avoids
this issue.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>