Vim: update jsx filetype detection

No need to check for "jsx" anymore as you no longer need "@jsx"
annotations in your code.

We just check for "React."

I also considered something like:

    if getline(n) =~ "\\v^\\s*var\\s+React\\s*\\="

which would check more narrowly (for `var React =`, or similar), but
let's start with something loose for now and see how we go.