correct unfinished phrase in the README
correct unfinished phrase in the README

Summary:
Short description of CSE in the README contains an unfinished phrase.
This corrects the phrase.

Reviewed By: MarcSzafraniec

Differential Revision: D26845514

fbshipit-source-id: de01c070b4aa091f8d362f32f02fc0e11b36e3d8

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2