Fix spec to be meaningful again

In @e900036, I changed a spec to make it pass without thinking about it.
This was pointed out in the
[code review](rubocop#4615 (review))
by @mikegee and I fixed it, but then somehow that thing got lost again, possibly
during rebasing. Sorry about that.

spec/rubocop/ast/send_node_spec.rb

@@ -539,9 +539,9 @@
 
   describe '#comparison_method?' do
     context 'with a comparison method' do
-      let(:source) { 'foo.bar <=> :baz' }
+      let(:source) { 'foo.bar >= :baz' }
 
-      it { expect(send_node.comparison_method?).to be_falsey }
+      it { expect(send_node.comparison_method?).to be_truthy }
     end
 
     context 'with a regular method' do