riscv: kexec_image: Fix dead link to boot-image-header.rst

Fix the reference to 'boot-image-header.rst', which was moved to
'Documentation/arch/riscv/' in commit 'ed843ae947f8'
("docs: move riscv under arch").

Signed-off-by: Soham Metha <sohammetha01@gmail.com>
Link: https://patch.msgid.link/20251203194355.63265-1-sohammetha01@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/kernel/kexec_image.c

@@ -22,7 +22,7 @@ static int image_probe(const char *kernel_buf, unsigned long kernel_len)
 	if (!h || kernel_len < sizeof(*h))
 		return -EINVAL;
 
-	/* According to Documentation/riscv/boot-image-header.rst,
+	/* According to Documentation/arch/riscv/boot-image-header.rst,
 	 * use "magic2" field to check when version >= 0.2.
 	 */