x86/boot/e820: Fix typo in e820.c comment

change "itsmain" to "its main".

Fixes: 544a0f47 ("x86/boot/e820: Rename e820_table_saved to e820_table_firmware and improve the description")
Signed-off-by: Wang Yong <yongw.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20221211103849.173870-1-yongw.kernel@gmail.com