Update Getting-started.md

without an additional npm i, you will get an error:
https://stackoverflow.com/questions/72181423/loopback-cli-lb-clean-command-not-found-linux

Signed-off-by: mauriceKA <post@patrik-spiess.de>

Author: mauriceKA

docs/site/Getting-started.md

@@ -63,6 +63,7 @@ running the project.
 
 ```sh
 cd getting-started
+npm i
 npm start
 ```