Fix typo in jet.h header comment

Replace y with 3 in f(1 + e, 3) to match the worked example on the
right-hand side.

Thanks to Nicolas Raynaud (@nraynaud) for reporting this.

Fixes #1198

Change-Id: I6fd893669d06991ea535f2e96f4677fb72428f02

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 50762e3..4768693 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -78,7 +78,7 @@
 //
 // For df/dx:
 //
-//   f(1 + e, y) = (1 + e)^2 + (1 + e) * 3
+//   f(1 + e, 3) = (1 + e)^2 + (1 + e) * 3
 //               = 1 + 2 * e + 3 + 3 * e
 //               = 4 + 5 * e
 //