Quote square brackets in shell commands.

This ensures compatibility with zsh.

Fix #2316.

CONTRIBUTING.md

@@ -121,7 +121,7 @@ Follow these steps to start contributing:
 4. Set up a development environment by running the following command in a virtual environment:
 
    ```bash
-   $ pip install -e .[dev]
+   $ pip install -e ".[dev]"
    ```
 
    (If transformers was already installed in the virtual environment, remove

README.md

@@ -120,14 +120,14 @@ Depending on which framework is installed (TensorFlow 2.0 and/or PyTorch), the i
 Here's the easiest way to run tests for the library:
 
 ```bash
-pip install -e .[testing]
+pip install -e ".[testing]"
 make test
 ```
 
 and for the examples:
 
 ```bash
-pip install -e .[testing]
+pip install -e ".[testing]"
 pip install -r examples/requirements.txt
 make test-examples
 ```

docs/README.md

@@ -4,7 +4,7 @@ To generate the documentation, you first have to build it. Several packages are
 you can install them with the following command, at the root of the code repository:
 
 ```bash
-pip install -e .[docs]
+pip install -e ".[docs]"
 ```
 
 ## Packages installed

src/transformers/commands/serving.py

@@ -109,7 +109,7 @@ class ServeCommand(BaseTransformersCLICommand):
         if not _serve_dependancies_installed:
             raise RuntimeError(
                 "Using serve command requires FastAPI and unicorn. "
-                "Please install transformers with [serving]: pip install transformers[serving]."
+                "Please install transformers with [serving]: pip install \"transformers[serving]\"."
                 "Or install FastAPI and unicorn separately."
             )
         else: