Improve instructions w.r.t. lunch setup in `test/README.chroot.md`.

Be more specific about the available lunch options in the "Set
lunch target and ADB" section of the ART Chroot-Based On-Device
Testing instructions.

Test: Render `README.chroot.md` as HTML and check the output manually
Bug: 34729697
Change-Id: I97444e683e84fbd0e0e99d5317993e9152bab13b
1 file changed