message: micro: Update person detection readme
  The README points to a Makefile target that no longer exists
  (test_person_detection_test).
  This commit changes the README file to instruct the developer to use the
  test_person_detection_test_int8 target instead.
diff: @@ -598,7 +598,7 @@ This will take a few minutes, and downloads frameworks the code uses like
  finished, run:
  ```
  -make -f tensorflow/lite/micro/tools/make/Makefile test_person_detection_test
  +make -f tensorflow/lite/micro/tools/make/Makefile test_person_detection_test_int8
  ```
  You should see a series of files get compiled, followed by some logging output
  