log: Fix comment for LOGC_BOOT

This comment is in the wrong format, so reports an error with
'make htmldocs'. Fix it.

Fixes: b73d61a5 ("x86: zimage: Add a little more logging")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>