godoc: added new Default values for SetConcurrency (#29)

it seems that the default was changed and the function documentation was wrong

gzip.go

@@ -69,8 +69,8 @@ type result struct {
 // With this you can control the approximate size of your blocks,
 // as well as how many you want to be processing in parallel.
 //
-// Default values for this is SetConcurrency(250000, 16),
-// meaning blocks are split at 250000 bytes and up to 16 blocks
+// Default values for this is SetConcurrency(defaultBlockSize, runtime.GOMAXPROCS(0)),
+// meaning blocks are split at 1 MB and up to the number of CPU threads
 // can be processing at once before the writer blocks.
 func (z *Writer) SetConcurrency(blockSize, blocks int) error {
 	if blockSize <= tailSize {