Fix port configuration in sitegen:serve #557

Job	Run time
checkstyle	29s
shellcheck	13s
build	1m 54s
copyright	17s
javadoc	1m 3s
tests/cli-linux	2m 1s
cli/linux-amd64	1m 48s
vscode-extension	1m 9s
spotbugs	2m 46s
cli/windows-amd64	3m 31s
tests/cli-windows	4m 0s
cli/darwin-amd64	4m 2s
tests/cli-functional-linux	3m 41s
tests/cli-functional-windows	5m 49s
tests/archetype-linux	2m 0s
tests/archetype-windows	3m 25s
tests/linker-linux	1m 38s
tests/linker-windows	3m 6s
tests/misc-linux	2m 25s
tests/misc-windows	4m 39s
	49m 56s

Provide feedback