fix: broken diagnostic redirects

Author: Ezrashaw

Diff for: book.toml

@@ -48,5 +48,6 @@ warning-policy = "error"
 
 [output.html.redirect]
 "/compiletest.html" = "tests/compiletest.html"
-"/diagnostics/sessiondiagnostic.html" = "diagnostics/diagnostic-structs.html"
+"/diagnostics/sessiondiagnostic.html" = "diagnostic-structs.html"
+"/diagnostics/diagnostic-codes.html" = "error-codes.html"
 "/miri.html" = "const-eval/interpret.html"