Fix small typo

calll -> call

Booting/linux-bootstrap-1.md

@@ -553,7 +553,7 @@ Jump to main
 That's all! We have the stack and BSS, so we can jump to the `main()` C function:
 
 ```assembly
-    calll main
+    call main
 ```
 
 The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/v4.16/arch/x86/boot/main.c). You can read about what this does in the next part.