docs: printk-formats: fix build warning

Add an empty line after the '::' starting the code block so that the
following lines are properly interpreted.

Without this, the following build warnings are visible.

Documentation/core-api/printk-formats.rst:136: WARNING: Unexpected indentation.
Documentation/core-api/printk-formats.rst:137: WARNING: Block quote ends without a blank line; unexpected unindent.

Fixes: 9294523e ("module: add printk formats to add module build ID to stacktraces")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210722100356.635078-2-ciorneiioana@gmail.com


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