fix doc of tcpsock:setclientcert api

Author: chronolaw

README.markdown

@@ -7742,7 +7742,7 @@ This method was first introduced in the `v0.5.0rc1` release.
 tcpsock:setclientcert
 --------------------
 
-**syntax:** *tcpsock:setclientcert(cert, pkey)*
+**syntax:** *ok, err = tcpsock:setclientcert(cert, pkey)*
 
 **context:** *rewrite_by_lua*, access_by_lua*, content_by_lua*, ngx.timer.*, ssl_certificate_by_lua*, ssl_session_fetch_by_lua*, ssl_client_hello_by_lua**