[doc] correct a typo in the parameter server example (#45927)

Asynchronous was mistakenly written as synchronous, I corrected it.

Signed-off-by: nono-Sang <1908865287@qq.com>

doc/source/ray-core/examples/plot_parameter_server.ipynb

@@ -324,7 +324,7 @@
    "source": [
     "## Asynchronous Parameter Server Training\n",
     "\n",
-    "We'll now create a synchronous parameter server training scheme. We'll first\n",
+    "We'll now create an asynchronous parameter server training scheme. We'll first\n",
     "instantiate a process for the parameter server, along with multiple\n",
     "workers."
    ]
@@ -414,4 +414,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 5
-}
+}