x86/boot/Documentation: Fix whitespace noise in boot.rst

There's a lot of unnecessary whitespace damage in this
file: space before tabs, etc., that has no formatting
or readability effect or advantages.

Fix them.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/176535283007.498.16442167388418039352.tip-bot2@tip-bot2