Fix typo in folder name

[0xpr03]

Docs mention add directory but show an adder folder in the tree.

[chriskrycho]

The text is correct, actually: the parent director is the add directory, and we are now creating a new binary package named adder within the add directory. That’s why we end up with the output we do from Cargo:

Adding `adder` as member of workspace at `file:///projects/add`

Thanks, though!