include <assert> -> <cassert>

Fixes reported public issue #280.

doc/index.html

@@ -22,7 +22,7 @@ directory.  The following example shows how to open a database,
 creating it if necessary:
 <p>
 <pre>
-  #include &lt;assert&gt;
+  #include &lt;cassert&gt;
   #include "leveldb/db.h"
 
   leveldb::DB* db;