full path required for extension install (#1670)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: ChristopherHX <[email protected]>

README.md

@@ -121,7 +121,7 @@ nix run nixpkgs#act
 Act can be installed as a [GitHub CLI](https://cli.github.com/) extension:
 
 ```sh
-gh extension install nektos/gh-act
+gh extension install https://github.com/nektos/gh-act
 ```
 
 ## Other install options