update readme with new stb_how.txt location

README.md

@@ -57,7 +57,7 @@ Yes. I'll put a list here when people remind me what they are.
 
 #### Do you have any advice on how to create my own single-file library?
 
-Yes. https://github.com/nothings/stb/blob/master/stb_howto.txt
+Yes. https://github.com/nothings/stb/blob/master/docs/stb_howto.txt
 
 #### Why public domain?