[XRay][docs] Update instructions

Add `xray_mode=xray-basic` to the list of options in the "further
exploration" section of the doc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340843 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed