Update performance.txt

Author: rustagir

source/fundamentals/performance.txt

@@ -90,7 +90,7 @@ For more information about the concepts in this guide, see the following pages:
 - :ref:`Connection Guide <rust-connect-to-mongodb-client>`
 
 API Documentation
------------------
+~~~~~~~~~~~~~~~~~
 
 - `Client() <{+api+}/struct.Client.html>`__