update selection vignette to fix #184

[zkamvar]

The function for pcadapt has changed:

• The input parameter is named "input" from "data"
• The data is expected to have variants in rows
• pcadapt will write temporary files to the user's working directory.

I have addressed these issues and made some minor grammer fixes.

[hlapp]

Other than the line comments I made, looks good.

[hlapp]

I'm a little unsure about leaving the temp file sitting around, but I'll go with your judgment that this likely be fixed upstream soon anyway.