Fix that dumb mistake in `testSong.md` doc

In [`testSong.md` doc file](https://github.com/gd-programming/gd.docs/blob/main/docs/endpoints/testSong.md), there is a dumb mistake.
The title:
**WARNING: THIS IS A GET REQUEST**
The code:
```requests.post(some_other_things)```
See what I mean ?