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

Documentation/core-api/printk-formats.rst

@@ -494,9 +494,11 @@ Time and date
 	%pt[RT]t		HH:MM:SS
 	%pt[RT][dt][r]
 
-For printing date and time as represented by
+For printing date and time as represented by::
+
 	R  struct rtc_time structure
 	T  time64_t type
+
 in human readable format.
 
 By default year will be incremented by 1900 and month by 1.