boot: Fix the reference to the kernel entrypoint

Booting/linux-bootstrap-5.md

@@ -483,7 +483,7 @@ Once the relocations are applied, the decompressor code jumps to the kernel entr
 
 Now we are in the kernel 🎉🎉🎉
 
-The kernel entrypoint is the `start_kernel` function from [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c). This is our next stop, but it will be in the next set of chapters - [Kernel initialization process](https://github.com/0xAX/linux-insides/tree/master/Initialization).
+The kernel entrypoint is the `startup_64` function from [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S). This is our next stop, but it will be in the next set of chapters - [Kernel initialization process](https://github.com/0xAX/linux-insides/tree/master/Initialization).
 
 ## Conclusion
 

Booting/linux-bootstrap-6.md

@@ -1,8 +1,8 @@
 # Kernel booting process. Part 6
 
-In the [previous part](./linux-bootstrap-5.md), we finally left the setup code and reached the Linux kernel itself. We explored the last steps of the early boot process - from the kernel decompression to the hand-off to the Linux kernel entrypoint (the `start_kernel` function). You may think this is the end of the set of posts about the Linux kernel booting process, but I'd like to come back one more time to the early setup code and look at one more important part of it - `KASLR` or Kernel Address Space Layout Randomization.
+In the [previous part](./linux-bootstrap-5.md), we finally left the setup code and reached the Linux kernel itself. We explored the last steps of the early boot process - from the kernel decompression to the hand-off to the Linux kernel entrypoint (the `startup_64` function). You may think this is the end of the set of posts about the Linux kernel booting process, but I'd like to come back one more time to the early setup code and look at one more important part of it - `KASLR` or Kernel Address Space Layout Randomization.
 
-As you can remember from the previous parts, the entry point of the Linux kernel is the `start_kernel` function defined in [main.c](https://github.com/torvalds/linux/blob/master/init/main.c). In normal cases, the kernel is loaded at the fixed, well-known address defined by the value of the `CONFIG_PHYSICAL_START` configuration option. The description and the default value of this option are defined in [arch/x86/Kconfig](https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig):
+As you can remember from the previous parts, the entry point of the Linux kernel is the `startup_64` function defined in [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S). In normal cases, the kernel is loaded at the fixed, well-known address defined by the value of the `CONFIG_PHYSICAL_START` configuration option. The description and the default value of this option are defined in [arch/x86/Kconfig](https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig):
 
 <!-- https://raw.githubusercontent.com/torvalds/linux/refs/heads/master/arch/x86/Kconfig#L2021-L2025 -->
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