add setclientcert link in readme

Author: chronolaw

README.markdown

@@ -263,7 +263,7 @@ in the current request before you reusing the `ctx` table in some other place.
 
 ## resty.core.socket.tcp
 
-* [tcpsock:tlshandshake](https://github.com/openresty/lua-nginx-module#tcpsocktlshandshake)
+* [tcpsock:setclientcert](https://github.com/openresty/lua-nginx-module#tcpsocksetclientcert)
 * [tcpsock:sslhandshake](https://github.com/openresty/lua-nginx-module#tcpsocksslhandshake)
 
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