Just follow the instructions in the video below!
Commands:
1. /execute @e[type=arrow] ~ ~ ~ setblock ~ ~ ~ fire
2. /kill @e[type=arrow]
Instructional Video
[video=youtube;JVqkUZgUmFg] https://www.youtube.com/watch?v=JVqkUZgUmFg [/video]
Whilst I love that you are experimenting with command blocks (Always great to see) I encourage you to find a better way to achieve what you already have. Currently with what you have created when you shoot a slab or carpet or any other non full block and late the arrow above it, it'll instantly delete the block. Also the command block is constantly trying to place fire throughout the air, try finding a way to check if the arrow is not in the air. Good luck and if you need any help feel free to message me!
I came up with this like in 1.0.5 and i have always done it the same way and now that you mention it there are better ways to do it.