update tls option (#46)

Author: jordan-smith721

source/configuration.txt

@@ -121,7 +121,7 @@ and certificate, and that has TLS enabled:
    #include <mongocxx/uri.hpp>
 
    auto client = mongocxx::client{
-       uri{"mongodb://host1/?authMechanism=MONGODB-X509&tlsCertificateFile=client.pem&tls=true"}
+       uri{"mongodb://host1/?authMechanism=MONGODB-X509&tlsCertificateKeyFile=client.pem&tls=true"}
        };
 
 See the MongoDB server :manual:`X.509 tutorial </tutorial/configure-x509-client-authentication/>`