docs(training): avoid an RST section underline in the DSpark loss docstring

The docs are built with MyST, where a line of dashes under text is a setext
H2. The generated apidocs page for this module therefore started at H2 and
tripped "Document headings start at H2, not H1 [myst.header]", which fails
the docs build because it treats warnings as errors. Use bold text instead;
this was the only RST underline under src/megatron/bridge.

Signed-off-by: khazic <khazzz1c@gmail.com>

Author: khazic

src/megatron/bridge/training/post_training/dspark/loss.py

@@ -28,8 +28,8 @@
 Acceptance is measured analytically as ``accept_rate = 1 - 0.5 * L1`` and the
 expected accepted prefix length of a block as ``tau = sum_k cumprod(accept_rate)_k + 1``.
 
-Return contract
----------------
+**Return contract**
+
 :func:`dspark_loss` returns the Megatron-Core 3-tuple ``(loss, num_tokens,
 report)``, matching :func:`megatron.bridge.training.losses.masked_next_token_loss`: