Fix typo in spec

R=golang-dev, rsc
https://golang.org/cl/164052

doc/go_spec.html

@@ -4402,7 +4402,7 @@ func generate(ch chan<- int) {
 	}
 }
 
-// Copy the values from channel 'in' to channel 'out',
+// Copy the values from channel 'src' to channel 'dst',
 // removing those divisible by 'prime'.
 func filter(src <-chan int, dst chan<- int, prime int) {
 	for i := range src {	// Loop over values received from 'src'.