Fix Problem::Evaluate documentation

Change-Id: I8c70a24743cff2d9cface99ef0f5d34c78f769c6

include/ceres/problem.h

diff --git a/include/ceres/problem.h b/include/ceres/problem.h
index b1ccbab..0a449cb 100644
--- a/include/ceres/problem.h
+++ b/include/ceres/problem.h
@@ -383,7 +383,7 @@
   //
   //   Problem problem;
   //   double x = 1;
-  //   problem.Add(new MyCostFunction, NULL, &x);
+  //   problem.AddResidualBlock(new MyCostFunction, NULL, &x);
   //
   //   double cost = 0.0;
   //   problem.Evaluate(Problem::EvaluateOptions(), &cost, NULL, NULL, NULL);