Fix a tutorial bug (#35)

db.cursor is a method, not a property.

Author: qizidog

docs/main.html

@@ -384,7 +384,7 @@ <h3>PooledDB (pooled_db)</h3>
 db.close()  # or del db</pre>
 <p>You can also use context managers for simpler code:</p>
 <pre class="literal-block">with pool.connection() as db:
-    with db.cursor as cur:
+    with db.cursor() as cur:
         cur.execute(...)
         res = cur.fetchone()</pre>
 <p>Note that you need to explicitly start transactions by calling the