Fix filename

Author: andre-richter

00_before_we_start/README.md

@@ -106,6 +106,6 @@ From a namespace perspective, **memory** subsystem code lives in:
 # Boot flow
 
 1. The kernel's entry point is the function `cpu::boot::arch_boot::_start()`.
-    - It is implemented in `src/_arch/__arch_name__/cpu/boot.rs`.
+    - It is implemented in `src/_arch/__arch_name__/cpu/boot.s`.