docs(cli): use ndjson code fence for TTY-refusal example (torrust#1798)

Use `ndjson` as the code fence language instead of `json` to prevent
the IDE's automatic JSON formatter from reformatting the single-line
example into a multi-line block on save.

Author: josecelano

docs/adrs/20260519000000_define_global_cli_output_contract.md

@@ -78,14 +78,11 @@ Rationale: when stdout is a TTY, result JSON would be mixed with the shell promp
 pipelines silently. Refusing makes the contract mechanically enforceable and the error
 immediately visible. Users can suppress the check with `| cat` or `| jq`.
 
-Example stderr record on TTY refusal (pretty-printed for readability; the actual wire format is
-a single JSON line per the NDJSON contract):
-
-```json
-{
-  "kind": "tty_refusal",
-  "message": "stdout is a TTY; pipe the output to consume result data"
-}
+Example stderr record on TTY refusal (one JSON object on a single line, as required by the
+NDJSON contract):
+
+```ndjson
+{"kind":"tty_refusal","message":"stdout is a TTY; pipe the output to consume result data"}
 ```
 
 ### 5. User-facing verbosity