Fix deprecation comment for Command.SetOutput (#2172)

Deprecation comments should be at the start of a paragraph [1], and because
of that have a whitespace above them [2];

> To signal that an identifier should not be used, add a paragraph to its
> doc comment that begins with Deprecated: followed by some information
> about the deprecation (...)

With the whitespace missing, some tools, including pkg.go.dev [3] don't
detect it to be deprecated.

[1]: https://go.dev/wiki/Deprecated
[2]: https://go.dev/doc/comment#paragraphs
[3]: https://pkg.go.dev/github.com/spf13/cobra@v1.8.1#Command.SetOutput

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>