utf8: fix typo.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7063056
diff --git a/src/pkg/unicode/utf8/utf8.go b/src/pkg/unicode/utf8/utf8.go
index ad23577..93d0be5 100644
--- a/src/pkg/unicode/utf8/utf8.go
+++ b/src/pkg/unicode/utf8/utf8.go
@@ -400,7 +400,7 @@
 		} else {
 			_, size := DecodeRune(p[i:])
 			if size == 1 {
-				// All valid runes of size of 1 (those
+				// All valid runes of size 1 (those
 				// below RuneSelf) were handled above.
 				// This must be a RuneError.
 				return false