update test to hook exist? rather than exists?

spec/tools/md5_lookup_spec.rb

@@ -311,7 +311,7 @@ def stub_load(with_setting=true)
         let(:opts) { subject.parse(valid_argv) }
 
         before(:example) do
-          allow(File).to receive(:exists?).and_return(true)
+          allow(File).to receive(:exist?).and_return(true)
         end
 
         it 'returns the input file path' do
@@ -330,7 +330,7 @@ def stub_load(with_setting=true)
 
       context 'when the required input file is not set' do
         before(:example) do
-          allow(File).to receive(:exists?).and_return(false)
+          allow(File).to receive(:exist?).and_return(false)
         end
 
         it 'raises an OptionParser::MissingArgument error' do