Updates reference to protobuf 4.24.0 issue with a url.

tensorboard/pip_package/requirements.txt

@@ -25,7 +25,7 @@ numpy >= 1.12.0
 # version to be installed that is also compatible with TensorFlow's constraints:
 # https://github.com/tensorflow/tensorflow/blob/25adc4fccb4b0bb5a933eba1d246380e7b87d7f7/tensorflow/tools/pip_package/setup.py#L101
 # 4.24.0 had an issue that broke our tests, so we should avoid that release:
-# protocolbuffers/protobuf#13485
+# https://github.com/protocolbuffers/protobuf/issues/13485
 protobuf >= 3.19.6, != 4.24.0
 setuptools >= 41.0.0  # Note: provides pkg_resources as well as setuptools
 # A dependency of our vendored packages. This lower bound has not been correctly