docs: fix API key format guidance to prevent common errors (modelcontextprotocol#1673)

* docs: fix API key format guidance to prevent common errors

* Update docs/docs/develop/build-client.mdx

Co-authored-by: Jonathan Hefner <[email protected]>

---------

Co-authored-by: Jonathan Hefner <[email protected]>

Author: YogeshK34

docs/docs/develop/build-client.mdx

@@ -77,7 +77,7 @@ You'll need an Anthropic API key from the [Anthropic Console](https://console.an
 Create a `.env` file to store it:
 
 ```bash
-echo "ANTHROPIC_API_KEY=<your key here>" > .env
+echo "ANTHROPIC_API_KEY=your-api-key-goes-here" > .env
 ```
 
 Add `.env` to your `.gitignore`: