docs/contributing: fix "ha256" -> "sha256" typo

doc/contributing/coding-conventions.chapter.md

···
     owner = "NixOS";
     repo = "nix";
     rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
-    hash = "ha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
+    hash = "sha256-7D4m+saJjbSFP5hOwpQq2FGR2rr+psQMTcyb1ZvtXsQ=";
   }
   ```