更新 chapter4.tex，参考答案标点

Chapter4/chapter4.tex

@@ -237,7 +237,7 @@
 \begin{example}
 机器译文：the the the the
 
-\qquad \ 参考答案：The cat is standing on the ground
+\qquad \ 参考答案：The cat is standing on the ground.
 \label{eg:4-bleu-example}
 \end{example}