fix category for "Second Book of Machine Language" (#2286)

Compute!'s "Second Book of Machine Language" by Richard Mansfield covers the MOS 6502 for Commodore, Apple, and Atari 8-bit systems. This moves it from the "Assembly Language" main section to the "Assembly Language Non-x86" subsection.

free-programming-books.md

@@ -751,7 +751,6 @@
 * [Ralf Brown's Interrupt List](http://www.ctyme.com/rbrown.htm)
 * [Software optimization resources](http://www.agner.org/optimize/) A. Fog
 * [The Art of Assembly Language Programming](https://courses.engr.illinois.edu/ece390/books/artofasm/artofasm.html)
-* [The Second Book Of Machine Language](http://www.atariarchives.org/2bml/)
 * [Wizard Code](http://vendu.twodots.nl/wizardcode.html)
 * [x86 Assembly](https://en.wikibooks.org/wiki/X86_Assembly)
 
@@ -762,6 +761,7 @@
 * [Machine Code for Beginners [Z80 and 6502 CPUs]](https://usborne.com/browse-books/features/computer-and-coding-books/) - Lisa Watts and Mike Wharton (PDF) *(Scroll to bottom and click on book cover)*
 * [Machine Language for Beginners](https://archive.org/details/ataribooks-machine-language-for-beginners) - Richard Mansfield [6502 CPU]
 * [Programmed Introduction to MIPS Assembly Language](http://chortle.ccsu.edu/AssemblyTutorial/index.html)
+* [The Second Book of Machine Language](http://www.atariarchives.org/2bml/)
 
 
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