fix code

https://github.com/golang-china/gopl-zh/issues/2

ch6/ch6-05.md

@@ -57,7 +57,7 @@ func (s *IntSet) String() string {
 				if buf.Len() > len("{") {
 					buf.WriteByte('}')
 				}
-				fmt.Fprintf(&buf, "%d", 64*i+j)"}")}}
+				fmt.Fprintf(&buf, "%d", 64*i+j)
 			}
 		}
 	}