fix simple trailing whitespace

fixes #742

https://bugs.ruby-lang.org/issues/7175

@bf4 @jeremy 
"It should be symmetric" is important, but symmetry implies more than the one check here. Options: 1. remove the new test that's not related to purpose of the commit; 2. make the spec name more precise to limit the scope of its claims; 3. expand the spec to actually demonstrate symmetry.