Minor fixes for proposalPlugin README
Minor fixes for proposalPlugin README

The README.md mentioned that "The proposalPlugin generates the following
two outputs:" even though it only generates one output named `rois`. Also
fixed two mismatches in parameter names in README and the code.

Signed-off-by: skarunaratne <ea96ea28981a6d27f5c2bf705d9a6578b2b1123f@nvidia.com>
Signed-off-by: Rajeev Rao <19c92b5a16bf20c576205f71d12c68705f41f65b@nvidia.com>

NVIDIA/TensorRT,NVIDIA/TensorRT,NVIDIA/TensorRT,NVIDIA/TensorRT,NVIDIA/TensorRT