fix(cli): rename the update command references to upgrade across the CLI and documentation

ts/systemd.ts

@@ -164,7 +164,7 @@ WantedBy=multi-user.target
           }`,
         );
         logger.log(
-          `  ${theme.dim('Run')} ${theme.command('sudo nupst update')} ${theme.dim('to upgrade')}`,
+          `  ${theme.dim('Run')} ${theme.command('sudo nupst upgrade')} ${theme.dim('to upgrade')}`,
         );
       } else {
         logger.log('');