Restore trailing newline (engine issue tracked in QuantEcon/action-translation#116)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/sir_model.md

@@ -413,4 +413,4 @@ paths = [path * ν * γ * pop_size for path in i_paths]
 plot_paths(paths, labels)
 ```
 
-如果我们能够将感染高峰推迟到疫苗研发出来之前，就有可能降低累计死亡人数。
+如果我们能够将感染高峰推迟到疫苗研发出来之前，就有可能降低累计死亡人数。