Fix Sphinx docstring error in Gemma4ConformerBlock

The `*0.5` in the forward flow docstring was parsed as RST inline
emphasis. Adding `::` makes the block a literal code block.

Author: yunchaoyang1 user

src/fairseq2/models/gemma4/audio/conformer.py

@@ -321,7 +321,8 @@ def forward(
 class Gemma4ConformerBlock(TransformerEncoderLayer):
     """Gemma4 conformer block.
 
-    Forward flow:
+    Forward flow::
+
       FFN1:  clamp -> pre_norm -> ffn -> clamp -> post_norm -> *0.5 -> residual
       Attn:  clamp -> pre_norm -> self_attn -> clamp -> post_norm -> residual
       Conv:  (mask) -> pre_norm -> conv -> residual