Fix typo in spec

R=golang-dev, rsc
https://golang.org/cl/164052
diff --git a/doc/go_spec.html b/doc/go_spec.html
index 8a24746..fc3fbaf 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -4402,7 +4402,7 @@
 	}
 }
 
-// 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'.