Add "abs tol" to an ODE doctest.

A "Computational Math..." book test fails for me with,

  Expected:
    [[0, 1], [0.5, 1.12419127424558], [1.0, 1.461590162288825]]
  Got:
    [[0, 1], [0.5, 1.12419127424558], [1.0, 1.4615901622888245]]

An "abs tol" fixes the issue backwards-compatibly.

Author: orlitzky

src/sage/tests/books/computational-mathematics-with-sagemath/integration_doctest.py

@@ -253,7 +253,7 @@
 Sage example in ./integration.tex, line 1363::
 
   sage: t, y = var('t, y')
-  sage: desolve_rk4(t*y*(2-y), y, ics=[0,1], end_points=[0, 1], step=0.5)
+  sage: desolve_rk4(t*y*(2-y), y, ics=[0,1], end_points=[0, 1], step=0.5)  # abs tol 1e-12
   [[0, 1], [0.5, 1.12419127424558], [1.0, 1.461590162288825]]
 
 Sage example in ./integration.tex, line 1399::