docs: fix apt-get install snippet in Ubuntu CI example

[joris-gallot]

Description

This PR fixes a small issue in the documentation for setting up Tauri dependencies on Ubuntu in GitHub Actions

Problem
The apt-get install -y line in the example was split across multiple lines without backslashes, causing each package name to be interpreted as a separate shell command. This led to errors like:

line 3: libwebkit2gtk-4.1-dev: command not found

Fix
Updated the snippet to install all required packages using backslashes for line continuation

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