Docs: Add missing import of 'ConfigurableField' in 'Full code comparison' example in LCEL (#15661)

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- **Description:** Add missing import of 'ConfigurableField' in 'Full
code comparison' example in LCEL
  - **Issue:** Example code not running
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  - **Twitter handle:** @heyyoshan

docs/docs/expression_language/why.ipynb

@@ -1007,7 +1007,7 @@
     "from langchain_openai import OpenAI\n",
     "from langchain_core.output_parsers import StrOutputParser\n",
     "from langchain_core.prompts import ChatPromptTemplate\n",
-    "from langchain_core.runnables import RunnablePassthrough\n",
+    "from langchain_core.runnables import RunnablePassthrough, ConfigurableField\n",
     "\n",
     "os.environ[\"LANGCHAIN_API_KEY\"] = \"...\"\n",
     "os.environ[\"LANGCHAIN_TRACING_V2\"] = \"true\"\n",