fix: loss_compare caption token count (20B -> 5B)

The loss_compare.png x-axis only spans 0-5B tokens. The caption
incorrectly claimed ~20B tokens.

main.tex

@@ -302,7 +302,7 @@ Sequence length & 2,048 tokens \\
 \centering
 \includegraphics[width=0.50\linewidth]{data/loss_compare.png}
 \caption{Training loss curves: GPT-2 residual scaling vs.~Kaiming
-initialization over $\sim$20B tokens.}
+initialization over $\sim$5B tokens.}
 \label{fig:loss}
 \end{figure}