Small change for the desktop entry spec

[Bastian, Waldo]

Thiago Macieira wrote:
> The UTF-8 encoding is recommended and the default, but it isn't
mandatory. 
> The Encoding= line can specify another encoding.

Not quite:
"The Encoding key is deprecated. It was used to specify whether keys of
type localestring were encoded in UTF-8 or in the specified locale.
Possible values are UTF-8 and Legacy-Mixed."

Cheers,
Waldo