Fix typo in comment section: .a[...] -> .v[...]

Change-Id: I2116645938c4552ed495e371bfdbcaf62a443461

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 74ce1e9..123b711 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -106,8 +106,8 @@
 //   Jet<double, 2> y(1);  // Pick the 1st dual number for y.
 //   Jet<double, 2> z = f(x, y);
 //
-//   LOG(INFO) << "df/dx = " << z.a[0]
-//             << "df/dy = " << z.a[1];
+//   LOG(INFO) << "df/dx = " << z.v[0]
+//             << "df/dy = " << z.v[1];
 //
 // Most users should not use Jet objects directly; a wrapper around Jet objects,
 // which makes computing the derivative, gradient, or jacobian of templated