Fix a number of typos in rotation.h

Thanks to @yiping for reporting these.
https://github.com/ceres-solver/ceres-solver/issues/672

Change-Id: I9c59d2b666430edf33f10aaba0d0ee4cd520e0a4

include/ceres/rotation.h

diff --git a/include/ceres/rotation.h b/include/ceres/rotation.h
index df2d75e..5107990 100644
--- a/include/ceres/rotation.h
+++ b/include/ceres/rotation.h
@@ -624,16 +624,16 @@
     result[2] = pt[2] * costheta + w_cross_pt[2] * sintheta + w[2] * tmp;
   } else {
     // Near zero, the first order Taylor approximation of the rotation
-    // matrix R corresponding to a vector w and angle w is
+    // matrix R corresponding to a vector w and angle theta is
     //
     //   R = I + hat(w) * sin(theta)
     //
     // But sintheta ~ theta and theta * w = angle_axis, which gives us
     //
-    //  R = I + hat(w)
+    //  R = I + hat(angle_axis)
     //
     // and actually performing multiplication with the point pt, gives us
-    // R * pt = pt + w x pt.
+    // R * pt = pt + angle_axis x pt.
     //
     // Switching to the Taylor expansion near zero provides meaningful
     // derivatives when evaluated using Jets.