uinput: fix a couple of errors in the README

* The link to example files in the CTS tests was actually pointing to
  example files for the hid command, not uinput. I can't find any for
  uinput.
* The example register command was missing EV_ABS from its UI_SET_EVBIT
  call.

Test: check rendering in Gitiles
Bug: 245989146
Flag: DOCS_ONLY
Change-Id: I6958a1ab17a380676ea7f78fd86ff3373ef194f2