x86, doc: fix references to Documentation/x86/i386/boot.txt

Impact: Documentation fix

The amazing dancing boot.txt file has jumped places again.  It should
never have been in Documentation/x86/i386, since it never was
32-bit-specific, but it unfortunately ended up there for a while.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>