Fix nested lists in CONTRIBUTING.md (#5721)

## Summary

We have a lot of two-space-indented stuff, but apparently it needs to be
four-space indented to render as expected in MkDocs.

CODE_OF_CONDUCT.md

@@ -6,10 +6,10 @@
 - [Scope](#scope)
 - [Enforcement](#enforcement)
 - [Enforcement Guidelines](#enforcement-guidelines)
-  - [1. Correction](#1-correction)
-  - [2. Warning](#2-warning)
-  - [3. Temporary Ban](#3-temporary-ban)
-  - [4. Permanent Ban](#4-permanent-ban)
+    - [1. Correction](#1-correction)
+    - [2. Warning](#2-warning)
+    - [3. Temporary Ban](#3-temporary-ban)
+    - [4. Permanent Ban](#4-permanent-ban)
 - [Attribution](#attribution)
 
 ## Our Pledge
@@ -33,20 +33,20 @@ community include:
 - Being respectful of differing opinions, viewpoints, and experiences
 - Giving and gracefully accepting constructive feedback
 - Accepting responsibility and apologizing to those affected by our mistakes,
-  and learning from the experience
+    and learning from the experience
 - Focusing on what is best not just for us as individuals, but for the
-  overall community
+    overall community
 
 Examples of unacceptable behavior include:
 
 - The use of sexualized language or imagery, and sexual attention or
-  advances of any kind
+    advances of any kind
 - Trolling, insulting or derogatory comments, and personal or political attacks
 - Public or private harassment
 - Publishing others' private information, such as a physical or email
-  address, without their explicit permission
+    address, without their explicit permission
 - Other conduct which could reasonably be considered inappropriate in a
-  professional setting
+    professional setting
 
 ## Enforcement Responsibilities