Docs: Update code structure doc

- fix the number of CPU to 8 to avoid Sphinx crash in OpenCI

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I4f05857a639ec2609f5b21cb739a6e91e5372fc0
diff --git a/docs/contributing/code_review_guide.rst b/docs/contributing/code_review_guide.rst
index 343fb17..622bf75 100644
--- a/docs/contributing/code_review_guide.rst
+++ b/docs/contributing/code_review_guide.rst
@@ -15,7 +15,7 @@
 
 - Read the :doc:`Contributing Process </contributing/contributing_process>`
   to know basic concepts.
-- Read the :doc:`Source Structure </design_docs/source_structure>`
+- Read the :ref:`source_structure`
   for structure related reference.
 
 The review guidelines consist of these items: