docs: Update README.md for rel 2.0.3.

README.md

@@ -46,7 +46,7 @@ changes may be written to a config file in a platform-specific location:
  this repository:
 
  ```sh
- $ go install decred.org/[email protected].2
+ $ go install decred.org/[email protected].3
  ```
 
  Using `@master` instead will perform a release build using the latest code