doc/code_rules: Replace block \code highlighters with ```c

Created by: AndyJGraham

We want to use the proper Syntax Highlighting for C-like code blocks and that also align with what we have in code_style.md.

Change-Id: Icd3c2b1d8978b704cd02665275f9f6523c00a58f Signed-off-by: Andrew Graham andrew.graham@arm.com