docs: use pnpm run plugins install in README

Etherpad uses pnpm internally. The canonical install command per
etherpad-lite/doc/plugins.md is `pnpm run plugins install ep_<name>`,
not `npm install`. Update README to match.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

README.md

@@ -9,7 +9,7 @@ Etherpad plugin to dynamically generate a pad's initial content.
 From the Etherpad working directory, run:
 
 ```shell
-npm install --no-save --legacy-peer-deps ep_dynamic_default_content
+pnpm run plugins install ep_dynamic_default_content
 ```
 
 Or, install from Etherpad's `/admin/plugins` page.