docs(weather-agent): tag MCP_URL code fence as text for MD040

Address review nit on PR #466: the env-var snippet inside the agent
import step had a bare ``` opener; markdownlint MD040 wants a language
tag. Tagging it `text` since it's a literal env var assignment, not a
shell command.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Mariusz Sabath <mrsabath@gmail.com>

Author: mrsabath

authbridge/demos/weather-agent/demo-ui-advanced.md

@@ -162,7 +162,7 @@ Now the UI flow (order matches the actual import form top-to-bottom):
 
    After import, **edit `MCP_URL`** in the variable list to point at the
    advanced tool service:
-   ```
+   ```text
    MCP_URL=http://weather-tool-advanced-mcp:8000/mcp
    ```
 10. **(Ollama only)** Expand **AuthBridge Advanced Configuration** and set