Moving colon.

Addressing:
  NameError: undefined local variable or method `report_file' for #<OpenSCAP::Xccdf::Session:0x00000001b0ba70>
/home/slukasik-f16/data/.hidden/workspace/ruby/ruby-openscap/lib/openscap/xccdf/session.rb:72:in `export_results'
/home/slukasik-f16/data/.hidden/workspace/ruby/ruby-openscap/test/xccdf/session_ds_test.rb:108:in `test_session_export_oval_variables'