Update a few references to "master" branches to "main".

PiperOrigin-RevId: 385651269

Author: jburnim

UNITTEST.md

@@ -118,7 +118,7 @@ del _DistributionTest  # Don't run tests for the base class.
 
 Helper class to test vector-event distributions.
 
-[VectorDistributionTestHelpers](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/contrib/distributions/python/ops/test_util.py#L193) ([Example Use](https://github.com/tensorflow/probability/blob/main/tensorflow_probability/python/distributions/mixture_same_family_test.py#L91))
+[VectorDistributionTestHelpers](https://github.com/tensorflow/probability/blob/main/tensorflow_probability/python/internal/test_util.py#L1085) ([Example Use](https://github.com/tensorflow/probability/blob/main/tensorflow_probability/python/distributions/mixture_same_family_test.py#L91))
 
 ## Testing Discrete, Scalar Distributions
 

tensorflow_probability/examples/jupyter_notebooks/TensorFlow_Probability_on_JAX.ipynb

@@ -1943,7 +1943,7 @@
     "id": "WOgYcS9zpJG3"
    },
    "source": [
-    "For more details about JAX's deterministic key splitting model, see [this guide](https://github.com/google/jax/blob/master/design_notes/prng.md)."
+    "For more details about JAX's deterministic key splitting model, see [this guide](https://github.com/google/jax/blob/main/design_notes/prng.md)."
    ]
   }
  ],

testing/run_github_tests.sh

@@ -36,7 +36,7 @@ fi
 
 install_bazel() {
   # Install Bazel for tests. Based on instructions at
-  # https://docs.bazel.build/versions/master/install-ubuntu.html#install-on-ubuntu
+  # https://docs.bazel.build/versions/main/install-ubuntu.html#install-on-ubuntu
   # (We skip the openjdk8 install step, since travis lets us have that by
   # default).