Vignettes errors: Add 'markdown' to Suggests: to fix it

[HenrikBengtsson]

When running revdep checks on doFuture, I spotted your recent R-devel check errors;

Version: 1.0.1
Check: re-building of vignette outputs
Result: ERROR
    Error(s) in re-building vignettes:
     ...
    --- re-building 'ExampleBinary.md' using md
    Error: processing vignette 'ExampleBinary.md' failed with diagnostics:
    there is no package called 'markdown'
    --- failed re-building 'ExampleBinary.md'

Not sure if this is the reason for ffdb884#diff-29a78bd2d26c7f8f10e9c28be08b2e024c452cee66d79fd19771d70ce39ce678, but please note that the solution to the above R CMD check --as-cran errors on recent R-devel version is to add the markdown package to Suggests:.

[xsswang]

Thank you for the finding. The new update switched to html version. Please try.