Fix typo in starlette import in README

Small docs fix for modelcontextprotocol#312.

The [actual code](https://github.com/comfuture/python-sdk/blob/d29e0e9ca33445285c04bc53bcebdda3b1986181/src/mcp/server/fastmcp/server.py#L22-L23) uses `starlette.routing` which is the correct package name.

Author: conorbranagan

README.md

@@ -357,7 +357,7 @@ You can mount the SSE server to an existing ASGI server using the `sse_app` meth
 
 ```python
 from starlette.applications import Starlette
-from starlette.routes import Mount, Host
+from starlette.routing import Mount, Host
 from mcp.server.fastmcp import FastMCP