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: Responsible ML Infra

README.md

@@ -47,7 +47,7 @@ https://pypi-nightly.tensorflow.org on Google Cloud. To install the latest
 nightly package, please use the following command:
 
 ```bash
-pip install -i https://pypi-nightly.tensorflow.org/simple fairness-indicators
+pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple fairness-indicators
 ```
 
 This will install the nightly packages for the major dependencies of Fairness

RELEASE.md

@@ -261,7 +261,7 @@
     the following command:
 
     ```
-    pip install -i https://pypi-nightly.tensorflow.org/simple fairness-indicators
+    pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple fairness-indicators
     ```
 
     Note: These nightly packages are unstable and breakages are likely to

tensorboard_plugin/README.md

@@ -36,7 +36,7 @@ https://pypi-nightly.tensorflow.org on Google Cloud. To install the latest
 nightly package, please use the following command:
 
 ```bash
-pip install -i https://pypi-nightly.tensorflow.org/simple tensorboard-plugin-fairness-indicators
+pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple tensorboard-plugin-fairness-indicators
 ```
 
 This will install the nightly packages for the major dependencies of Tensorboard

tensorboard_plugin/tensorboard_plugin_fairness_indicators/RELEASE.md

@@ -250,7 +250,7 @@
     the following command:
 
     ```
-    pip install -i https://pypi-nightly.tensorflow.org/simple tensorboard-plugin-fairness-indicators
+    pip install --extra-index-url https://pypi-nightly.tensorflow.org/simple tensorboard-plugin-fairness-indicators
     ```
 
     Note: These nightly packages are unstable and breakages are likely to