Fix the test name in readme (#1878)
Summary:
`test_bench.py` only supports eager mode and we don't have plan to make it support pt2 mode.
Fixes https://github.com/pytorch/benchmark/issues/1850
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1878
Reviewed By: aaronenyeshi
Differential Revision: D49025049
Pulled By: xuzhao9
fbshipit-source-id: 0ad2dbf7b14db36e3b7edc5282d6b41550421d54
