Rename RungeKutta to RungeKuttaU

This could be done with lambda instead, but we'll leave it.  I need a
variant where time gets passed in and this was using that call
signature.  I'd rather rename this one.

Change-Id: Ic8e473db599528e17efc0dc30a734679237a5cca
diff --git a/frc971/control_loops/runge_kutta_test.cc b/frc971/control_loops/runge_kutta_test.cc
index aa64638..abd9466 100644
--- a/frc971/control_loops/runge_kutta_test.cc
+++ b/frc971/control_loops/runge_kutta_test.cc
@@ -26,7 +26,7 @@
   ::Eigen::Matrix<double, 1, 1> y0;
   y0(0, 0) = 0.0;
 
-  ::Eigen::Matrix<double, 1, 1> y1 = RungeKutta(
+  ::Eigen::Matrix<double, 1, 1> y1 = RungeKuttaU(
       [](::Eigen::Matrix<double, 1, 1> x, ::Eigen::Matrix<double, 1, 1> u) {
         ::Eigen::Matrix<double, 1, 1> y;
         y(0, 0) = ::std::exp(u(0, 0) * x(0, 0));