Close unclosed files (#735)

* Cose unclosed files

* Close additional unclosed file

* Dont use defer in loop

* Removed missed instances of defer in loop

eBook/exercises/chapter_12/cat_numbered.go

@@ -39,5 +39,6 @@ func main() {
 			continue
 		}
 		cat(bufio.NewReader(f))
+		f.Close()
 	}
 }