docs: fix references to code of conduct document (#6879)

substrate/README.md

@@ -15,7 +15,7 @@ also try out one of the [tutorials](https://substrate.dev/en/tutorials).
 
 ## Contributions & Code of Conduct
 
-Please follow the contributions guidelines as outlined in [`docs/CONTRIBUTING.adoc`](docs/CONTRIBUTING.adoc). In all communications and contributions, this project follows the [Contributor Covenant Code of Conduct](docs/CODE_OF_CONDUCT.adoc).
+Please follow the contributions guidelines as outlined in [`docs/CONTRIBUTING.adoc`](docs/CONTRIBUTING.adoc). In all communications and contributions, this project follows the [Contributor Covenant Code of Conduct](docs/CODE_OF_CONDUCT.md).
 
 ## Security
 

substrate/docs/README.adoc

@@ -523,7 +523,7 @@ include::CONTRIBUTING.adoc[]
 
 === Contributor Code of Conduct
 
-include::CODE_OF_CONDUCT.adoc[]
+include::CODE_OF_CONDUCT.md[]
 
 == License