Fix copy-pasted test name to reflect behavior

Author: nwoythal

spec/unit/puppet-lint/plugins/check_whitespace/trailing_whitespace_spec.rb

@@ -106,10 +106,10 @@
       end
     end
 
-    context 'empty line with nothing but whitespace' do
+    context 'empty lines with nothing but whitespace' do
       let(:code) { " \n " }
 
-      it 'only detects a single problem' do
+      it 'detects problems with both empty lines' do
         expect(problems).to have(2).problem
       end