docs(cli): update --format json example to match the new field order

The struct-layout fix in internal/output.jsonDiagnostic (previous
commit) reordered Go struct fields to group pointer-containing fields
before scalars, which changes encoding/json's emitted key order. This
reference page's --format json example still showed the pre-fix
order. Verified against actual `mdsmith check --format json` output.

Found by an independent adversarial review pass on PR #765.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SNiF3CVZG2NCizbsTgur4V

Author: claude

docs/reference/cli.md

@@ -154,13 +154,13 @@ path (`····^`) pointing to the exact column.
 [
   {
     "file": "README.md",
-    "line": 10,
-    "column": 81,
     "rule": "MDS001",
     "name": "line-length",
     "severity": "error",
     "message": "line too long (120 > 80)",
     "source_lines": ["Previous line.", "Another context.", "The long line...", "Next.", "Another."],
+    "line": 10,
+    "column": 81,
     "source_start_line": 8
   }
 ]