doc: Better to update the system before registering the client against Landscape Server

[nobuto-m]

landscape-documentation/docs/how-to-guides/landscape-installation-and-set-up/install-landscape-client.md

Lines 10 to 14 in 8d5bd31

	1. Install the `landscape-client` package:
	```bash
	sudo apt update && sudo apt install -y landscape-client
	```

There are some limitations that it takes time to synchronize installed/available package information when the system is not up to date. To maximize the smoothness of the initial Landscape experience, it would be good to make sure to run apt upgrade for example.

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Document: getting-started-with-landscape.md