[5.1.x] Refs #32193 -- Updated python-memcached to pymemcache in contributing guide.

Follow up to 05f3a61.
Backport of 337c641 from main

Author: felixxm

docs/internals/contributing/writing-code/unit-tests.txt

@@ -325,7 +325,7 @@ dependencies:
 * :pypi:`pywatchman`
 * :pypi:`redis` 3.4+
 * :pypi:`setuptools`
-* :pypi:`python-memcached`, plus a `supported Python binding
+* :pypi:`pymemcache`, plus a `supported Python binding
   <https://memcached.org/>`_
 * `gettext <https://www.gnu.org/software/gettext/manual/gettext.html>`_
   (:ref:`gettext_on_windows`)