Fix syntax in Dangerfile

Author: robertodr

Dangerfile

@@ -18,5 +18,5 @@ if git.commits.any? { |c| c.message =~ /^Merge branch '#{github.branch_for_base}
   fail('Please rebase to get rid of the merge commits in this PR')
 end
 
-code_style_validation.check validator: ['yapf'],
-                            file_extensions: ['.py'] 
+code_style_validation.check validator: 'yapf',
+                            file_extensions: ['.py']