Improve wiki content width. (#4195)

public/less/_markdown.less

@@ -5,6 +5,10 @@
     line-height: 1.6 !important;
     word-wrap: break-word;
 
+    &.ui.segment {
+        padding: 3em;
+    }
+
     &.file-view {
         padding: 2em 2em 2em !important;
     }