Close unclosed files (#735)

* Cose unclosed files

* Close additional unclosed file

* Dont use defer in loop

* Removed missed instances of defer in loop

eBook/examples/chapter_12/cat.go

@@ -31,5 +31,6 @@ func main() {
 			continue
 		}
 		cat(bufio.NewReader(f))
+		f.Close()
 	}
 }