Update README.md (EleutherAI#2297)

* Update README.md

I encounter some Git buffer size limits when trying to download all commits history of the repository, such as:
```error: RPC failed; curl 18 transfer closed with outstanding read data remaining
error: 5815 bytes of body are still expected
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF```

therefore the installation is faster and there are not errors when I download only the last version of the repository

* Fix linting issue

README.md

@@ -54,7 +54,7 @@ The Language Model Evaluation Harness is the backend for 🤗 Hugging Face's pop
 To install the `lm-eval` package from the github repository, run:
 
 ```bash
-git clone https://github.com/EleutherAI/lm-evaluation-harness
+git clone --depth 1 https://github.com/EleutherAI/lm-evaluation-harness
 cd lm-evaluation-harness
 pip install -e .
 ```