docs: multierr.Errors returns nil, not empty (#33)

Previously the docs incorrectly stated that `multierr.Errors(err)`
would return an empty slice rather than a nil slice, which would imply
that `multierr.Errors(nil) == []error{}`, which isn't true.

This updates the docs to reflect reality: `multierr.Errors(nil) == nil`.

error.go

@@ -130,7 +130,7 @@ type errorGroup interface {
 }
 
 // Errors returns a slice containing zero or more errors that the supplied
-// error is composed of. If the error is nil, the returned slice is empty.
+// error is composed of. If the error is nil, a nil slice is returned.
 //
 // 	err := multierr.Append(r.Close(), w.Close())
 // 	errors := multierr.Errors(err)