Update list of dev colors (but correctly)

In the `dev` preset, success codes (2xx) are [colored green](https://github.com/expressjs/morgan/blob/master/index.js#L193). This change updates the documentation to reflect that.

README.md

@@ -112,7 +112,7 @@ Standard Apache common log output.
 
 Concise output colored by response status for development use. The `:status`
 token will be colored red for server error codes, yellow for client error
-codes, cyan for redirection codes, green for `200`, and uncolored for all other codes.
+codes, cyan for redirection codes, green for success codes, and uncolored for information codes.
 
 ```
 :method :url :status :response-time ms - :res[content-length]