Fix make_new_version.sh

Summary:
One of the commands in the script make_new_version.sh was
incorrect. Fixed it.

Test Plan: Try the script out to verify that it works.

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34791

build_tools/make_new_version.sh

@@ -37,7 +37,7 @@ fi
 
 title "Adding new tag for this release ..."
 BRANCH="$ROCKSDB_VERSION.fb"
-$GIT co -b $BRANCH
+$GIT checkout -b $BRANCH
 
 # Setting up the proxy for remote repo access
 title "Pushing new branch to remote repo ..."