docs: Update README.md

- This is not pnpm@latest way to format that command
- Fixed syntax for pnpm installation

Author: MichalBryxi

README.md

@@ -20,7 +20,7 @@ Installation
 
 #### pnpm
 ```bash
-pnpm add --dev @ember/test-helpers
+pnpm add --save-dev @ember/test-helpers
 ```
 
 #### npm