docs: Memory and RAG: add missing backtick for class reference (#6656)

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## Why are these changes needed?
Update `Memory and RAG` doc to include missing backticks for class
references.
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solves. -->

## Related issue number

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Co-authored-by: Victor Dibia <victor.dibia@gmail.com>

Author: roysha1

python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/memory.ipynb

@@ -20,7 +20,7 @@
     "\n",
     "## ListMemory Example\n",
     "\n",
-    "{py:class}~autogen_core.memory.ListMemory is provided as an example implementation of the {py:class}~autogen_core.memory.Memory protocol. It is a simple list-based memory implementation that maintains memories in chronological order, appending the most recent memories to the model's context. The implementation is designed to be straightforward and predictable, making it easy to understand and debug.\n",
+    "{py:class}`~autogen_core.memory.ListMemory` is provided as an example implementation of the {py:class}`~autogen_core.memory.Memory` protocol. It is a simple list-based memory implementation that maintains memories in chronological order, appending the most recent memories to the model's context. The implementation is designed to be straightforward and predictable, making it easy to understand and debug.\n",
     "In the following example, we will use ListMemory to maintain a memory bank of user preferences and demonstrate how it can be used to provide consistent context for agent responses over time."
    ]
   },