fixing comment

Author: ieliz

demos/interactive_face_detection_demo/cpp_gapi/README.md

@@ -69,7 +69,7 @@ omz_converter --list models.lst
 
 Running the demo with `-h` shows this help message:
 
-```sh
+```
     [ -h]                                         show the help message and exit
     [--help]                                      print help on all arguments
       -m <MODEL FILE>                             path to an .xml file with a trained Face Detection model