Avoid spacing between ! and its argument

Addressing rubocop comment.

Author: isimluk

test/xccdf/session_test.rb

@@ -37,6 +37,6 @@ def test_session_new_nil
 
   def test_sds_false
     @s = OpenSCAP::Xccdf::Session.new('../data/xccdf.xml')
-    assert ! @s.sds?
+    refute @s.sds?
   end
 end