```Fix typo in AutoDiffManifold comment and docs

Change-Id: I0b50e3e10661578e68f713e34a59f65b3692e745
```
```diff --git a/docs/source/nnls_modeling.rst b/docs/source/nnls_modeling.rst
index 71d31e0..8b84066 100644
--- a/docs/source/nnls_modeling.rst
+++ b/docs/source/nnls_modeling.rst
```
```@@ -1715,7 +1715,7 @@
x_plus_delta = Plus(x, delta);
y_minus_x    = Minus(y, x);

-Where, ``x``, ``y`` and ``x_plus_y`` are vectors on the manifold in
+Where, ``x``, ``y`` and ``x_plus_delta`` are vectors on the manifold in
the ambient space (so they are ``kAmbientSize`` vectors) and
``delta``, ``y_minus_x`` are vectors in the tangent space (so they are
``kTangentSize`` vectors).
```
```diff --git a/include/ceres/autodiff_manifold.h b/include/ceres/autodiff_manifold.h
index 09b0aa2..4bf7e56 100644
--- a/include/ceres/autodiff_manifold.h
+++ b/include/ceres/autodiff_manifold.h
```
```@@ -47,8 +47,8 @@
//   x_plus_delta = Plus(x, delta);
//   y_minus_x    = Minus(y, x);
//
-// Where, x, y and x_plus_y are vectors on the manifold in the ambient space (so
-// they are kAmbientSize vectors) and delta, y_minus_x are vectors in the
+// Where, x, y and x_plus_delta are vectors on the manifold in the ambient space
+// (so they are kAmbientSize vectors) and delta, y_minus_x are vectors in the
// tangent space (so they are kTangentSize vectors).
//
// The Functor should have the signature:
```