Recommend xvfb-run -d instead of xvfb-run -a

[micheljung]

Fixes #270

[antoinerg]

Thank you @micheljung for submitting a PR 😸

In light of #270 (comment), I think we should keep mentioning the -a flag which is necessary for older versions of xvfb-run (which are still prevalent). However, mentioning that users should use -d for newer versions of xvfb-run is a great addition.

[antoinerg]

@micheljung do you mind me taking over this PR and make the appropriate modifications?

[micheljung]

@antoinerg not at all, go ahead :)