vim: rename `singlethread` to `syncscope`

SVN r307722 renamed the keyword from `singlethread` to `syncscope`.
Update the syntax file accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328211 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed