Update SUMMARY.md

Author: SamyPesse

SUMMARY.md

@@ -9,6 +9,7 @@
 * [GDT](Chapter-6/README.md)
 * [IDT and interrupts](Chapter-7/README.md)
 * [Memory management: physical and virtual](Chapter-8/README.md)
+* Memory management: physical and virtual
 * Process management and multitasking
 * External program execution: ELF files
 * Userland and syscalls
@@ -20,5 +21,4 @@
 * Standard C library (libC)
 * UNIX basic tools: sh, cat
 * Lua interpreter
-* Memory management: physical and virtual