doc: fix downloading kernel for ARM instances

Also fix for when the patch kernel version is <100.

Signed-off-by: Pablo Barbáchano <[email protected]>

Author: pb8o

docs/getting-started.md

@@ -100,7 +100,7 @@ kernel image with a Ubuntu 24.04 rootfs from our CI:
 ```bash
 ARCH="$(uname -m)"
 
-latest=$(wget "http://spec.ccfc.min.s3.amazonaws.com/?prefix=firecracker-ci/v1.10/x86_64/vmlinux-5.10&list-type=2" -O - 2>/dev/null | grep "(?<=<Key>)(firecracker-ci/v1.10/x86_64/vmlinux-5\.10\.[0-9]{3})(?=</Key>)" -o -P)
+latest=$(wget "http://spec.ccfc.min.s3.amazonaws.com/?prefix=firecracker-ci/v1.10/$ARCH/vmlinux-5.10&list-type=2" -O - 2>/dev/null | grep -oP "(?<=<Key>)(firecracker-ci/v1.10/$ARCH/vmlinux-5\.10\.[0-9]{1,3})(?=</Key>)")
 
 # Download a linux kernel binary
 wget "https://s3.amazonaws.com/spec.ccfc.min/${latest}"