src/sage/functions/piecewise.py: switch one test from giac to sympy

There's one piecewise test that uses algorithm='giac', but it will
soon be possible to install sage without giac. We could make the test
conditional on giac's presence, but the same thing works with sympy,
so let's just use that.

src/sage/functions/piecewise.py

@@ -828,9 +828,9 @@ def integral(self, parameters, variable, x=None, a=None, b=None, definite=False,
             Check that the algorithm keyword can be used::
 
                 sage: ex = piecewise([([0, 1], 1), ((1, oo), 1/x**2)])
-                sage: integral(ex, x, 0, 100, algorithm='giac')
+                sage: integral(ex, x, 0, 100, algorithm='sympy')
                 199/100
-                sage: integral(ex, x, algorithm='giac')
+                sage: integral(ex, x, algorithm='sympy')
                 piecewise(x|-->x on [0, 1], x|-->-1/x + 2 on (1, +oo); x)
             """
             if a is not None and b is not None: