docs: core-api/printk-formats.rst: use literal block syntax

Fix the following warning:

WARNING: Definition list ends without a blank line;
unexpected unindent.

By switching to the literal block syntax.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-8-dwlsalmeida@gmail.com


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