fixing npm install

npm install n8n g does not work and lead to several issues on fs and patch-package:

https://community.n8n.io/t/n8n-install/39983
n8n-io/n8n-nodes-starter#45

I found npx works like a charm so better to just use it here too as it is meant for new commers.

_snippets/integrations/creating-nodes/testing.md

@@ -2,7 +2,7 @@ You can test your node as you build it by running it in a local n8n instance.
 
 1. Install n8n using npm:
   ```shell
-  npm install n8n -g
+  npx n8n
   ```
 2. When you are ready to test your node, publish it locally:
   ```shell