Fix installation instructions (#54)

The Heroku CLI no longer automatically expands plugin names
like `api` to their long form package name.

As such, the old install instructions in the README now result
in a random package named `api` being installed from npmjs.org,
and that package is not a Heroku CLI plugin (!!).

Fixes #53.

README.md

@@ -7,7 +7,7 @@
 ## How to install this plugin
 
 ```shell
-$ heroku plugins:install api
+$ heroku plugins:install @heroku-cli/plugin-api
 ```
 
 ## Usage