docs(readme): use v0.3.0

Author: pyk

README.md

@@ -7,7 +7,7 @@ CLI testing for Zig.
 1. Fetch the latest release:
 
     ```shell
-    zig fetch --save=cmdtest https://github.com/pyk/cmdtest/archive/v0.2.0.tar.gz
+    zig fetch --save=cmdtest https://github.com/pyk/cmdtest/archive/v0.3.0.tar.gz
     ```
 
     This updates `build.zig.zon`.