update doc

Author: wenli.lw

README.md

@@ -32,7 +32,7 @@ $ fire ./test.js arg1 arg2
  $ fire mathjs add 5.1 5.2
  ```
 
-3. [node-open](https://github.com/josdejong/mathjs)
+3. [node-open](https://github.com/pwnall/node-open)
 
  ```
  $ npm install open