Update install instructions to match new package name (#36)

Author: karl-run

README.md

@@ -137,7 +137,7 @@ This module is distributed via [npm][npm] which is bundled with [node][node] and
 should be installed as one of your project's `devDependencies`:
 
 ```
-npm install --save-dev svelte-testing-library
+npm install --save-dev @testing-library/svelte
 ```
 
 This library has `peerDependencies` listings for `svelte`.