Correct a docstring typographical error

Author: rocky

mathics/builtin/numbers/algebra.py

@@ -372,7 +372,7 @@ class Apart(Builtin):
     >> Apart[1 / (x^2 + 5x + 6)]
      = 1 / (2 + x) - 1 / (3 + x)
 
-    When several variables are involved, the results can be different
+    When several variables are involved, the results can be different \
     depending on the main variable:
     >> Apart[1 / (x^2 - y^2), x]
      = -1 / (2 y (x + y)) + 1 / (2 y (x - y))