src/sage/symbolic/expression.pyx: Fix bad use of line continuation

Author: Matthias Koeppe

src/sage/symbolic/expression.pyx

@@ -238,7 +238,7 @@ Check that :trac:`9880` is fixed::
     11/27*b_0/((2^b_2)^2*(2^b_1)^2*(2^b_0)^2) - \
     11/27*b_1/((2^b_2)^2*(2^b_1)^2*(2^b_0)^2) - \
     11/27*b_2/((2^b_2)^2*(2^b_1)^2*(2^b_0)^2) + \
-    64/81/((2^b_2)^2*(2^b_1)^2*(2^b_0)^2) + 35/81 \
+    64/81/((2^b_2)^2*(2^b_1)^2*(2^b_0)^2) + 35/81
     sage: f.nops()
     38