docs: fix broken documentation links

Mostly due to x86 and acpi conversion, several documentation
links are still pointing to the old file. Fix them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>
Reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

arch/x86/boot/header.S

@@ -313,7 +313,7 @@ start_sys_seg:	.word	SYSSEG		# obsolete and meaningless, but just
 
 type_of_loader:	.byte	0		# 0 means ancient bootloader, newer
 					# bootloaders know to change this.
-					# See Documentation/x86/boot.txt for
+					# See Documentation/x86/boot.rst for
 					# assigned ids
 
 # flags, unused bits must be zero (RFU) bit within loadflags