Fix typo in build environment name (#38343)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/38343
Test Plan: `grep pytorch-linux-xenial-py3.6-gcc5.4-test .circleci -R` + CI
Differential Revision: D21537546
Pulled By: malfet
fbshipit-source-id: 4e790eaee388e51e28640b43d56ef2c07ca146c4
