[doc] normalize HF Transformers string
I noticed we use  🤗 Transformer instead of 🤗 Transformers in a couple of places, this PR fixes it.

That was a tricky one with UTF32 emoji. This worked.

```
LC_ALL=C find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|(\xF0\x9F\xA4\x97 Transformer) |$1s |' {} \;
```
To grep I had to do:

```
LC_ALL=C grep -Ir -P '\xF0\x9F\xA4\x97 Transformer ' .
```

@sgugger 
