Fix "does not return a subclass instance" test for Time#-

This looks like a copy-pase from Time#+

Author: herwinw

core/time/minus_spec.rb

@@ -109,7 +109,7 @@
 
   it "does not return a subclass instance" do
     c = Class.new(Time)
-    x = c.now + 1
+    x = c.now - 1
     x.should be_an_instance_of(Time)
   end