[Docs] group-exports: fix syntax highlighting

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First chunk: https://share.cleanshot.com/spXGCRRGJkpBsLGLPk7k
Second chunk: https://share.cleanshot.com/vhRsmDnxCd7PZfFfLLhh

I searched codebase for any other ```flow js code blocks,
but there were only these two from this file:
https://share.cleanshot.com/xY7W9TJTXPrJ8ngtf6Kr

docs/rules/group-exports.md

@@ -62,7 +62,7 @@ test.another = true
 module.exports = test
 ```
 
-```flow js
+```ts
 const first = true;
 type firstType = boolean
 
@@ -105,7 +105,7 @@ module.exports.first = true
 module.exports.second = true
 ```
 
-```flow js
+```ts
 type firstType = boolean
 type secondType = any