Fixes the drop-down issue when book summary is displayed.

The original line is to fixes sighingnow#56, but actually this issue
was fixed by e758bc8 (for sighingnow#70).

Resolves sighingnow#93.

Signed-off-by: Tao He <sighingnow@gmail.com>

assets/gitbook/custom.css

@@ -14,7 +14,6 @@
     position: fixed;
     width: calc(100% - 300px);
     overflow-x: hidden;
-    overflow-y: hidden;
 }
 
 .book-body .body-inner {