Fix Jet lerp test regression

Partially revert changes from 1d5aff059cc2f95f6a1d3a42e2b41736fb386f81
to those in 8426526dff146a141c59eb581689ee249e146209.

Fixes #775

Change-Id: I6b2f481521f15bf09c039283e79f9ee13664f987

internal/ceres/jet_test.cc

diff --git a/internal/ceres/jet_test.cc b/internal/ceres/jet_test.cc
index ef05f40..44db7aa 100644
--- a/internal/ceres/jet_test.cc
+++ b/internal/ceres/jet_test.cc
@@ -647,7 +647,7 @@
   EXPECT_THAT(lerp(y, y, J{0}), IsAlmostEqualTo(y));
   EXPECT_THAT(lerp(x, y, J{0.5}), IsAlmostEqualTo((x + y) / J{2.0}));
   EXPECT_THAT(lerp(x, y, J{2}), IsAlmostEqualTo(J{2.0} * y - x));
-  EXPECT_THAT(lerp(x, y, J{-2}), IsAlmostEqualTo(J{3.0} * x - J{2} * x));
+  EXPECT_THAT(lerp(x, y, J{-2}), IsAlmostEqualTo(J{3.0} * x - J{2} * y));
 }
 
 TEST(Jet, Midpoint) {