Fix a type in interfacing_with_autodiff.html

Thanks to @tangobravo for reporting this.

https://github.com/ceres-solver/ceres-solver/issues/701

Change-Id: I2b063267c9b501c698ce6cd439a666363ba16fde

docs/source/interfacing_with_autodiff.rst

diff --git a/docs/source/interfacing_with_autodiff.rst b/docs/source/interfacing_with_autodiff.rst
index 02f58b2..e7bcfd7 100644
--- a/docs/source/interfacing_with_autodiff.rst
+++ b/docs/source/interfacing_with_autodiff.rst
@@ -37,7 +37,7 @@
    template <typename T> T TemplatedComputeDistortion(const T r2) {
      const double k1 = 0.0082;
      const double k2 = 0.000023;
-     return 1.0 + k1 * y2 + k2 * r2 * r2;
+     return 1.0 + k1 * r2 + k2 * r2 * r2;
    }
 
    struct Affine2DWithDistortion {