Use http link instead of ftp

The ftp:// protocol is not recognized as a link.

free-programming-books.md

@@ -1533,7 +1533,7 @@ See also [TeX](#tex)
 
 
 ###Scheme
-* [An Introduction to Scheme and its Implementation](ftp://ftp.cs.utexas.edu/pub/garbage/cs345/schintro-v14/schintro_toc.html)
+* [An Introduction to Scheme and its Implementation](http://www.cs.rpi.edu/academics/courses/fall00/ai/scheme/reference/schintro-v14/schintro_toc.html)
 * [Concrete Abstractions: An Introduction to Computer Science Using Scheme](https://gustavus.edu/+max/concrete-abstractions.html)
 * [How to Design Programs](http://htdp.org)
 * [Simply Scheme: Introducing Computer Science](http://www.cs.berkeley.edu/~bh/ss-toc2.html)