fix: correctly invoke uv run

This was found by @rohitkhatana in modelcontextprotocol#169

Author: dsp-ant

quickstart/server.mdx

@@ -311,7 +311,7 @@ Make sure you pass in the absolute path to your server.
 
 This tells Claude for Desktop:
 1. There's an MCP server named "weather"
-2. To launch it by running `uv --directory /ABSOLUTE/PATH/TO/PARENT/FOLDER/weather run weather`
+2. To launch it by running `uv --directory /ABSOLUTE/PATH/TO/PARENT/FOLDER/weather run weather.py`
 
 Save the file, and restart **Claude for Desktop**.
 </Tab>