Fix a typo in cubic_interpolation.h

https://github.com/ceres-solver/ceres-solver/issues/439

Change-Id: Ib8c5b2511dd8b162784f387f1624d8568b3a57df

include/ceres/cubic_interpolation.h

diff --git a/include/ceres/cubic_interpolation.h b/include/ceres/cubic_interpolation.h
index a26697b..4ff868a 100644
--- a/include/ceres/cubic_interpolation.h
+++ b/include/ceres/cubic_interpolation.h
@@ -116,7 +116,7 @@
 // Example usage:
 //
 //  const double data[] = {1.0, 2.0, 5.0, 6.0};
-//  Grid1D<double, 1> grid(x, 0, 4);
+//  Grid1D<double, 1> grid(data, 0, 4);
 //  CubicInterpolator<Grid1D<double, 1>> interpolator(grid);
 //  double f, dfdx;
 //  interpolator.Evaluator(1.5, &f, &dfdx);