Updated docs to reflect change in optional zstd dependency from zstandard to backports.zstd (#3721)

* Update docs to reflect change in optional zstd dependency from `zstandard` to `backports.zstd`

This was done in 93e6ae2, but the docs were not updated.

Signed-off-by: Edgar Ramírez Mondragón <[email protected]>

* Drop unneeded changelog entry

---------

Signed-off-by: Edgar Ramírez Mondragón <[email protected]>
Co-authored-by: Illia Volochii <[email protected]>

Author: edgarrmondragon

docs/advanced-usage.rst

@@ -562,7 +562,7 @@ Zstandard Encoding
 is a compression algorithm created by Facebook with better compression
 than brotli, gzip and deflate (see `benchmarks <https://facebook.github.io/zstd/#benchmarks>`_)
 and is supported by urllib3 in Python 3.14+ using the `compression.zstd <https://peps.python.org/pep-0784/>`_ standard library module
-and for Python 3.13 and earlier if the `zstandard package <https://pypi.org/project/zstandard/>`_ is installed.
+and for Python 3.13 and earlier if the `backports.zstd package <https://pypi.org/project/backports.zstd/>`_ is installed.
 You may also request the package be installed via the ``urllib3[zstd]`` extra:
 
 .. code-block:: bash