Documentation: coding_style: Add *long* to long multi-line comment example

Update the example, after the short multi-line comment alternative was
added several years ago, when the Wiki was still used.

Change-Id: I401180a9ac7c7cdc45fb8e9ba364823092cea6da
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88492
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

Documentation/contributing/coding_style.md

@@ -522,7 +522,7 @@ The preferred style for *long* (multi-line) comments is:
 
 ```c
 /*
- * This is the preferred style for multi-line
+ * This is the preferred style for long multi-line
  * comments in the coreboot source code.
  * Please use it consistently.
  *