doc: Fix up another broken code example in tls.rst
doc: Fix up another broken code example in tls.rst

class Server_Credentials is not defined in namespace Botan either.

Signed-off-by: Liu Qun <d52da801d1fb609d2c81e923996dc2d8723ef7ad@qq.com>
webmaster128/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan,randombit/botan,webmaster128/botan,randombit/botan,webmaster128/botan,randombit/botan