CODING_STYLE: Fix non-uniform heading level

Heading uses only 3 level header (###) but the rest of the file uses
four (####).  We should be uniform.

Use uniform heading level in line with the rest of the file.

Signed-off-by: Tobin C. Harding <me@tobin.cc>

CODING_STYLE.md

@@ -382,7 +382,7 @@ rules to use them:
   }
   ```
 
-### Cast to `(void)` When Intentionally Ignoring Return Values
+#### Cast to `(void)` When Intentionally Ignoring Return Values
 
 - There are cases where you do not care about the return value of a function.
   Please cast the return value to `(void)` when doing so.