bytes: make Buffer comment more accurate

Change-Id: Ief22b3dbba9616dd40bf3ea8e2633d3c5e7d1886
Reviewed-on: https://go-review.googlesource.com/19761
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/buffer.go

@@ -17,7 +17,7 @@ import (
 type Buffer struct {
 	buf       []byte            // contents are the bytes buf[off : len(buf)]
 	off       int               // read at &buf[off], write at &buf[len(buf)]
-	runeBytes [utf8.UTFMax]byte // avoid allocation of slice on each WriteByte or Rune
+	runeBytes [utf8.UTFMax]byte // avoid allocation of slice on each call to WriteRune
 	bootstrap [64]byte          // memory to hold first slice; helps small buffers (Printf) avoid allocation.
 	lastRead  readOp            // last read operation, so that Unread* can work correctly.
 }