Remove as typo from callbacks.h

Change-Id: I69fdae40adaed812454d7ff4d7e8909a5539e304

internal/ceres/callbacks.h

diff --git a/internal/ceres/callbacks.h b/internal/ceres/callbacks.h
index 29a3d32..288d6ae 100644
--- a/internal/ceres/callbacks.h
+++ b/internal/ceres/callbacks.h
@@ -58,7 +58,7 @@
  public:
   GradientProblemSolverStateUpdatingCallback(int num_parameters,
                                              const double* internal_parameters,
-                                             double* user_parameters_);
+                                             double* user_parameters);
   virtual ~GradientProblemSolverStateUpdatingCallback();
   virtual CallbackReturnType operator()(const IterationSummary& summary);
  private: