Fix markdown syntax highlighting for babel-flow (#6231)

Previously, the "Copy markdown" button in the playground generated
markdown containing this when having selected "babel-flow" as parser:

    ```babel-flow

This commit changes it to:

    ```jsx

So that the code blocks get syntax highlighting on GitHub.

website/playground/markdown.js

@@ -37,6 +37,7 @@ function getMarkdownSyntax(options) {
   switch (options.parser) {
     case "babel":
     case "babylon": // backward compatibility
+    case "babel-flow":
     case "flow":
       return "jsx";
     case "typescript":