Documentation: boot.rst: fix warnings

Fix WARNING: Inline emphasis start-string without end-string.
This warning was due to wrong syntax being used.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20191223033121.1584930-1-dwlsalmeida@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>