Fix challenge 74: add missing Examples section and fix JAX starter comment

- Add required <h2>Example</h2> section to challenge.html (was missing,
  checklist requires Implementation Requirements, Example(s), Constraints)
- Fix starter.jax.py comment: "on the GPU" → "on GPU" to match CLAUDE.md
  JAX template format

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]

challenges/medium/74_gpt2_block/challenge.html

@@ -119,6 +119,13 @@
   <li><strong>Residual 2:</strong> \(\text{output} = x' + F\)</li>
 </ol>
 
+<h2>Example</h2>
+<p>With <code>seq_len</code> = 4, <code>x</code> has shape (4, 768) and <code>output</code> has shape (4, 768).
+The <code>weights</code> buffer packs all 7,087,872 block parameters in the order described in the
+Weight Layout section below. Applying the transformer block transforms each token embedding through
+layer norm, multi-head self-attention with residual, layer norm, feed-forward network with residual,
+producing a new (4, 768) embedding tensor.</p>
+
 <h2>Implementation Requirements</h2>
 <ul>
   <li>Use only native features (external libraries are not permitted)</li>

challenges/medium/74_gpt2_block/starter/starter.jax.py

@@ -2,7 +2,7 @@
 import jax.numpy as jnp
 
 
-# x, weights are tensors on the GPU
+# x, weights are tensors on GPU
 @jax.jit
 def solve(x: jax.Array, weights: jax.Array, seq_len: int) -> jax.Array:
     # return output tensor directly