Update Alchemy Key URL (langchain-ai#3559)

Update Alchemy Key URL in Blockchain Document Loader. I want to say
thank you for the incredible work the LangChain library creators have
done.

I am amazed at how seamlessly the Loader integrates with Ethereum
Mainnet, Ethereum Testnet, Polygon Mainnet, and Polygon Testnet, and I
am excited to see how this technology can be extended in the future.

@hwchase17 - Please let me know if I can improve or if I have missed any
community guidelines in making the edit? Thank you again for your hard
work and dedication to the open source community.

docs/modules/indexes/document_loaders/examples/blockchain.ipynb

@@ -43,7 +43,7 @@
         "\n",
         "\n",
         "*   An OpenAI key (for OpenAI models)\n",
-        "*   A free [Alchemy API Key](https://https://www.alchemy.com/)\n",
+        "*   A free [Alchemy API Key](https://www.alchemy.com/)\n",
         "\n",
         "\n",
         "\n"