pacman: update install instructions

Adding the `-u` flaf to install according to best practices

https://github.com/tldr-pages/tldr/pull/1562

pages/linux/pacman.md

@@ -6,9 +6,9 @@
 
 `pacman -Syu`
 
-- Install a new package:
+- Install a given package and update all other packages to their latest versions according to the local database:
 
-`pacman -S {{package_name}}`
+`pacman -Su {{package_name}}`
 
 - Remove a package and its dependencies: