Fixed incorrect argument name in RotationMatrixToQuaternion()

- Raised as: https://github.com/ceres-solver/ceres-solver/pull/607 by
  Frank Dellaert

Change-Id: Id3e9f190e814cf18206e2f8c3b1b67b995c21dd5

include/ceres/rotation.h

diff --git a/include/ceres/rotation.h b/include/ceres/rotation.h
index 7d5c8ef..0c82a41 100644
--- a/include/ceres/rotation.h
+++ b/include/ceres/rotation.h
@@ -320,8 +320,8 @@
 }
 
 template <typename T>
-void RotationMatrixToQuaternion(const T* R, T* angle_axis) {
-  RotationMatrixToQuaternion(ColumnMajorAdapter3x3(R), angle_axis);
+void RotationMatrixToQuaternion(const T* R, T* quaternion) {
+  RotationMatrixToQuaternion(ColumnMajorAdapter3x3(R), quaternion);
 }
 
 // This algorithm comes from "Quaternion Calculus and Fast Animation",