docs: fix stale command reference in README.md

* npm run install-plugin -> npm run install-extension

To reflect the actual script name in package.json

Author: bowlofarugula

README.md

@@ -90,4 +90,4 @@ To install from source, follow these steps:
 * Clone the repository: `git clone https://github.com/bytecodealliance/vscode-wit.git && cd vscode-wit`
 * Install build dependencies: `npm run setup-wasm`
 * Run npm commands to install:
-`npm ci && npm run install-plugin`
+`npm ci && npm run install-extension`