Fix wrong flag for Pypi nightly repository

We should use `--extra-index-url` to specify extra index for Python packages, instead of `-i`.

PiperOrigin-RevId: 448149226

Author: tf-transform-team

README.md

@@ -68,7 +68,7 @@ Google Cloud. To install the latest nightly package, please use the following
 command:
 
 ```bash
-pip install -i https://pypi-nightly.tensorflow.org/simple tensorflow-transform
+pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple tensorflow-transform
 ```
 
 This will install the nightly packages for the major dependencies of TFT such

RELEASE.md

@@ -486,7 +486,7 @@
     following command:
 
     ```
-    pip install -i https://pypi-nightly.tensorflow.org/simple tensorflow-transform
+    pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple tensorflow-transform
     ```
 
     Note: These nightly packages are unstable and breakages are likely to

docs/install.md

@@ -64,7 +64,7 @@ TFT also hosts nightly packages at https://pypi-nightly.tensorflow.org on Google
 Cloud. To install the latest nightly package, please use the following command:
 
 ```
-pip install -i https://pypi-nightly.tensorflow.org/simple tensorflow-transform
+pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple tensorflow-transform
 ```
 
 This will install the nightly packages for the major dependencies of TFT such as