Fixed typo in thrid party checkout script

Author: bluemarvin

tools/taskcluster/get_third_party.sh

@@ -5,4 +5,4 @@ cp tools/ssh/config ~/.ssh
 python tools/taskcluster/fetch_secret.py -s project/firefoxreality/github-deploy-key -o ~/.ssh/deploymentkey_rsa -n key
 chmod go-rwx ~/.ssh/deploymentkey_rsa
 git clone git@github.com:MozillaReality/FirefoxReality-android-third-party.git third_party
-pushd third_party && git co -b target-branch `cat ../third_party_hash` && popd
+pushd third_party && git checkout -b target-branch `cat ../third_party_hash` && popd