themes: allow automatic last update

Fixes #45

Author: changkun

content/errata.md

@@ -74,7 +74,3 @@ title: 勘误表
 - 表 13-1 不同等级的浪费”下方算式错误
 
 <img width="868" alt="image" src="https://user-images.githubusercontent.com/7698088/164685097-518683e5-8a8b-4f6b-9e04-5f2952d8cea9.png">
-
-
----
-最后更新：2022.05.13

themes/book/layouts/_default/baseof.html

@@ -79,6 +79,12 @@
 {{ define "main" }}
   <article class="markdown">
     {{- .Content -}}
+    {{ $path := .Permalink | relURL }}
+    {{ if eq $path "/go-questions/errata/" }}
+    ---
+    <br/>
+    最后更新：{{ now.Format "2006.01.02" }}
+    {{ end }}
   </article>
 {{ end }}